A new procedure for spectrophotometric determination of bacteriolytic activity in biological fluids is described. The method usesMicrococcus lysodeikticuscells as substrate. By inactivation of the sample through pre‐incubation in ice‐water, a large number of samples can be prepared and analysed simultaneously. The necessity of performing the whole analysis in the photometer for one sample at the time is thus eliminated. By measuring the increase in transmission at 570 nm after incubation of the samples at 37 C a relatively long reaction time and wide concentration range is obtained. This makes the results quite precise and reproducible. The method has been used to determine the bacteriolytic activity in serum of healthy adults. Men have significantly higher levels than wo