The effect of a synthetic pyrethroid - alpha-cypermethrin administered per os for 28 days to Swiss mice was examined on phagocytic and bactericidal activity of neutrophils, and leukocytic image, IL-12 p70 level in blood plasma, as well as histologic and ultrastructural picture of the liver, heart, kidneys, lung and spleen. A synthetic pyrethroid alpha-cypermethrin, [(R,S)-alpha-cyano-3-phenoxybenzyl (R,S)-cis,trans-3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropanecarboxylate], produced by the Chemical Plant in Jaworzno was used in the study. The preparation for the application per os was used in doses 1/2 LD(50) (25 mg/kg body mass) and 1/5 LD(50) (10 mg/kg body mass). The results were presented as mean (x) +/- standard error (SEM) and subjected to statistical analysis by the parametric t-Student test. Subacute poisoning of mice with alpha-cypermethrin in doses 1/2 LD(50) and 1/5 LD(50) resulted in decreased bactericidal activity of neutrophils. The dose 10 mg/kg body mass had a stronger stimulatory effect on phagocytic activity than 25 mg/kg body mass. Significantly higher numbers of monocytes and lymphocytes were observed in the blood of male mice poisoned with 1/5 LD(50) alpha-cypermethrin. The administration of alpha-cypermethrin resulted for both doses in the decrease in IL-12 p70 serum secretion. The lowest IL-12 p70 level (pg/ml) was noted among female mice administered 1/2 LD(50) of the preparation. The results of the study may indicate that the pyrethroid in the study had a suppressive effect on Il-12 p70 production. In mice administered 1/5 LD(50) or 1/2 LD(50) of the preparation examined, histopathologic and ultrastructural changes were observed in the liver and kidneys.