Rabbits were given dialy doses of aflatoxin B1 equivalents of 0.03, 0.05, 0.07, and 0.09 mg for a 2-week period. Macrophages were harvested at the end of the experimental period, and in vitro phagocytosis experiments were conducted using Aspergillus fumigatus spores as ingestion particles. Alveolar macrophages from rabbits given the above doses of aflatoxin had reduced phagocytic activity when compared with macrophages from control rabbits. Incorporation of serum from the aflatoxin-treated rabbits in the in vitro culture system resulted in less stimulation of phagocytosis by macrophages from control rabbits when compared with the same system incorporating control serum. Stimulation of phagocytosis by macrophages was least when both serum and macrophages from aflatoxin-treated rabbits were used in the in vitro system.