The effect of the toxic substances of diphtheria corynebacteria (diphtheria toxin, diphtheria anatoxin, and codivac) on the aggregation of human platelets in vitro was demonstrated using platelet-rich plasma prepared from citrated blood and the standard platelet activator ADP (2• -5 M). These substances induce platelet aggregation in a dosedependent manner. Incubation of diphtheria toxin and anatoxin with platelets reduces ADP-induced and total platelet aggregation, the effect being dependent on the inducer dose and incubation time. By contrast, codivac stimulates ADP-induced and total platelet aggregation in all experimental series.