All cephalosporins studied have been shown to give hydrogen sulphide. The yield of hydrogen sulphide varies widely for different cephalosporins, but is highly reproducible for individual cephalosporins. The hydrogen sulphide formed can be converted into methylene blue directly in the degraded solution, and this procedure has been made the basis of a calorimetric method of determining cephalosporins. Penicillins do not give hydrogen sulphide under these conditions and the method is thus selective for cephalosporins. The manual visible spectrophotometric method for the determination of cephalosporins by alkaline degradation to sulphide and formation of methylene blue has been adapted for use with an air-segmented Autoanalyser I system. The system has been tested as a method of determining trace amounts of cephalosporins and other sulphideproducing impurities in penicillin G and penicillin V samples. The detection limit was calculated to be 1–2 μg.g-1 of cephalosporin in penicillin samples.