Details are given of the protein and casein yields during the complete lactation of healthy cows maintained under conditions of reasonably uniform management at the experimental farms of the Institute of Animal Genetics. Methods of chemical analysis are stated in detail, with especial reference to the precipitation of the casein at its isoelectric point. The standard errors involved in the chemical work were determined experimentally and two methods for precipitating the casein were compared without finding a substantial difference. An examination is made of the effect of various 'environmental conditions on the secretion of protein. Age of the cow was found to have a definite effect on the amount of protein secreted; it was found to increase with age. Service period also affects the amount, the longer the S.P. the bigger the amount of protein. Cows calving in October and November give the highest yields and those calving in March and April give the lowest. A dry period preceding the lactation has no effect if it exceeds three weeks. Statistical methods used at present to investigate the problems of inheritance of the milk yield and its constituents are discussed in detail. The squared difference method is employed and certain difficulties in its application are noted, requiring an adjustment of one of the raw figures to compensate for the fact that one relationship group is unduly dominated by the progeny of a single bull. The results show that the amounts of protein and casein secreted during lactation are inherited. The effect of the sire was found to be equal to that of the dam in the transmission of the genes governing the inheritance of these two characters. The mean square difference between the yields of unrelated cows was greater than the mean square difference between the yields of any other relationship group. The mean square difference between the yields of dam and daughter was also found. Arising from this study certain correlations between the protein yield of related cows are included.