Serum, plasma, lysed blood cells, urine and pleural fluid or lymph (inflammatory exudate from subcutaneous inoculation) were obtained from cattle naturally and artificially infected with Mycoplasma mycoides. These fluids were examined for antigens of M. mycoldes by means of the agar gel double diffusion precipitin test and the quantitative agar gel precipitin test. They were also examined for antibodies against M. mvcoides by means of the complement fixation and slide agglutination tests, and for viable M. tavcoides by growth in broth cultures. Hyperiimuune sera for use in these tests were prepared in sheep and cattle by the Intravenous injection of washed organisms that had been grown in broth medium. From the natural cases of CBPP viable organisms were obtained from the pleural fluid only, while in the experimental cases the organisms were present in the serum, plasma and lymph. Antibodies against M. mycoldes were demonstrated in sera and plasma samples of all cases. Specific precipitating antigens were demonstrated in all the fluids, urine possessing at least 5 serologically distinct antigens, lymph and pleural fluid at least 6 and serum and plasma at least 6 and sometimes 7 antigens. The 5 in urine were common to all fluids, while the extra 1 in lymph and pleural fluid was also present in serum and plasma. In addition to these so-called major antigens, minor ones, at least 6 in number, were also demonstrated, but these were apparently primarily associated with the organisms. The major precipitating antigens were predominantly extracellular with only small amounts present in the organisms. These major antigens were also elaborated by the organisms when grown In artificial culture medium, and those produced by fully virulent organisms were apparently identical to those produced by avirulent organisms. Fractionation of pooled urine from the artificially infected cattle by precipitation with varying volumes of cold iso-propyl alcohol and deproteinizatlon with a chlorofona-butanol mixture was undertaken. A total of 6 serologically distinct precipitating antigens were demonstrated in the AGT and separation of these antigens was possible to a limited extent by varying the volumes of alcohol used. Fraction C/l/2/3, the fraction which contained all the antigens, was shown to contain approximately 5.6 per cent. Kjeldahl N, 0.5 per cent. P., 42.4 per cent, carbohydrate (estimated as galactose) and 11.9 per cent, hexosamine. One or possibly 2 of the precipitin bands was shown to contain lipid, but there was no indication of nucleic acid. By the use of paper chromatography, galactose was demonstrated and probably sorbose and arahinose, together with some amino acids. These antigens were resistant to a temperature of 94°C. for 60 minutes and to the action of trypsin. Separation of the individual antigens was not obtained by either ultracentrifugation or electrophoresis. The antigenic fraction fixed complement in the presence of hyperimmune sheep sera, and fraction C/l/2/3 absorbed 87.5 and 96.9 per cent, of the agglutinating antibodies and 93.75 and 87.5 per cent, of the complement fixing antibodies from hyperimmune sheep x and sheep 6 sera respectively. This fraction was pyrogenic in rabbits and relatively non toxic to cattle, rabbits and mice, but proved to be lethal to fowl embryos. The antigens were haptens in cattle, rabbits and mice, but precipitating antibodies were produced in rabbits when the antigens were combined with "shiga conjugated protein". Fraction C/l/2/3 possessed an aggressive action when inoculated together with viable M. mycoides in immune animals and appeared to enhance the virulence of the organisms in susceptible cattle. It is suggested that these antigens play a part in assisting the growth of jf. mycoldes in the host tissues but are not in themselves significantly harmful.