1. We evaluated the ability of human colostrum adhering cells to phagocytize sheep red blood cells (E) incubated with rabbit anti-E IgG antibody (A) and zymosan particles incubated with fresh human serum or with the aqueous phase of colostrum. 2. The cells were found to have considerably intense phagocytic ability, i.e., 96.8% phagocytized EA particles, 83.2% phagocytized zymosan particles opsonized with fresh human serum, and 73.3% phagocytized zymosan particles opsonized with the aqueous phase of colostrum. Thus, the aqueous phase of colostrum can opsonize zymosan particles, an activity attributed to the complement system. 3. Total hemolytic complement (CH50) and the C3 component in a pool of normal human serum were two-fold higher than in a pool of the aqueous phase of colostrum. 4. These results indicate the existence of Fc gamma and C3 receptors on the membrane of human colostrum macrophages and suggest that these cells may be biologically active.