In cytotoxicity testing with murine lymphocytes, cytolysis, i.e. the complete loss of cell contours of the damaged and stained lymphocytes as a result of fluid influx, can cause considerable false negative titre determinations, as demonstrated by time-lapse microphotography. Under the usual test conditions, significant typing errors may be excpeted, if protein free diluents are used for titrating H-2 antisera and/or complement. From the kinetics of cytotoxicity and cytolysis at different BSA concentrations and antibody dilutions, an optimal cytotoxicity assay is proposed with special regard to the protein concentration and the time elapsing between the staining of the lymphocytes and the determination of cytotoxicity.