Purified aflatoxin B1 (AFB1) or AFB1 plus aflatoxin B2 (AFB2) was given daily for 5 weeks in gelatin capsules to 2-week-old feather-sexed broilers. In Experiment 1, pure AFB1 was given in doses equivalent to the quantity of toxin received, if diets containing either 0, 200, 500 or 1000 ppb of AFB1 were consumed. In Experiment 2, pure AFB1 or AFB1 plus B2 was administered in capsules in doses equivalent to the quantity of toxin received, if diets containing either 0, 100, 200, or 400 ppb of AFB1 were consumed. In Experiment 1, pure AFB1 greater than or equal to 500 ppb was only mildly toxic. These levels produced a significant decrease in the 5-week weight gain and microscopic lesions indicative of alfatoxicosis. No morbidity, mortality, or effects on feed conversion or immune responses, however, were noted in birds given pure AFB1 at these levels. Gross liver lesions indicative of aflatoxin toxicity occurred at the 1000 ppb only. Results of Experiment 2 were similar to the first. Weight gain and feed conversion were not affected for broilers receiving pure AFB1 as low as 200 ppb. No morbidity, mortality, or gross lesions were evident in birds given either pure AFB1 or AFB1 plus AFB2 as high as 400 ppb. However, cell-mediated immunity as measured by a delayed hypersensitive skin test was significantly affected in birds receiving 400 ppb AFB1 plus AFB2. No effects on humoral immunity or the development of acquired immunity to Newcastle disease or fowl cholera vaccination were noted.