MACROPHAGE ACTIVITY WAS STUDIED IN RATS INFECTED WITH TRYPANOSOMA LEWISI IN THREE PROTOCOL GROUPS: one group was fed complete diet, a second group was given a vitamin B(12)-deficient diet, and a third group was fed a pair-fed control (calorically restricted) diet. Throughout the observational period, in animals fed complete and pairfed diets, marked increases in acid phosphatase levels in peritoneal macrophages were directly related to the degree of parasitemia. Acid phosphatase levels in rats deprived of vitamin B(12) were approximately one third that of animals with an adequate supply of the vitamin. Irrespective of the diets, the infection with T lewisi also elicited increased macrophage phagocytosis of polystyrene latex particles and macrophage spreading. Both of these activities occurred at a much slower rate in the vitamin B(12)-deficient animals.