The cytotoxic effect of serum on guinea pig thymocytes was studied by dye exclusion and by analysis of cell volume distribution curves obtained by means of a computer-assisted electronic cell volume distribution analyzer. Guinea pig, rabbit and human sera were shown to be cytotoxic for guinea pig thymocytes (incubation for 1 h at 37 degrees C or overnight at 4 degrees C). A similar, but less potent, activity was found in autologous guinea pig serum (4 degrees C). The activity was dialysable and lyophilizable but was destroyed by heating at 56 degrees C for 30 min. It was recovered after ammonium sulphate precipitation followed by dialysis. The effect of this dialysate was dose-dependent. A tentative definition of viable and dead cells was made on the basis of 'electronic' cell volume. The following results indicate that by this definition electronic cell volume distribution allowed detection of the cytotoxic serum factor: (1) sera and dialysates produced marked distortion of cell volume distribution curves, not seen in controls treated with heat-inactivated preparations; (2) the dose-response relationship was similar when dye exclusion and cell volume distribution was used as assay: (3) both methods detected similar differences in sensitivity of different thymocyte subpopulations to serum; (4) the electronic method could be used on separate thymocyte subpopulations although they differed in cell volume. This rapid, objective and reproducible method for studying the cytotoxic effect of sera may be useful during attempts to purify and identify the active factor(s).