The toxicologic properties of the synthetic pyrethroid insecticide cypermethrin have been studied in animal experiments. In the course of acute and subchronic experiments performed by the administration of 1/2 and 1/40, 1/20, and 1/10 proportions of the oral LD50 value, no considerable changes were found in the general toxicologic tests. The compound known to affect the central nervous system, however, induced only a mild enhancement of the central excitation level shown by the EEG investigations in the doses applied. In the immunotoxicologic studies an early and dose-dependent suppression was induced as a result of humoral immune response of rabbits immunised by S. typhi following the administration of cypermethrin. The cell-mediated immune response was also decreased. In rats the immune response determined by anti-sheep erythrocyte and antiovalbumin titer as well as by autologous rosette formation of spleen lymphocytes was hindered. The earliest detectable group of symptoms indicating the effect of a mild cypermethrin exposure is the appearance of the positivity of the immunotoxicologic tests. On the basis of the experimental findings the sensitive immunotoxicologic tests have been regarded as of great importance in the series of toxicologic methods.