Effects of the chemical carcinogen 20-methylcholanthrene (MC) on toad Bufo marinus natural killer (NK) cells were tested in vivo and in vitro. Toads fed with MC (5 mg toad-1 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 .mu.g ml-1) at room temperature for 1 h, and afterwards washed 3 times, had lowered NK activity (47 to 85%). 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.