ABSTRACT. Effects of the chemical carcinogen 20-methylcholanthrene (MC) on toad Bufo marinus natural luller (NK) cells were tested in vivo and in vitro. Toads fed with MC (5 mg toad-' once per wk) mixed with olive oil exhibited an initial increase in NK activity at 10 d, followed by a suppression (60 %) at 20 and 30 d compared to control toads fed only olive oil. Conjugate formation was examined and demonstrated a slight decrease. Leukocytes which were preincubated with MC (5 to 100 pg ml-') at room temperature for 1 h, and afterwards washed 3 times, had lowered NK activity (47 to 85 Ye). In conclusion, the observed inhibition of NK cell activity by MC may predispose toads to carcinogenesis by a mechanism similar to that which occurs In mammals.