Effects of orchiectomy on male Calomys callosus infected with the Y strain of Trypanosoma cruzi were studied. Male C. callosus of the same age and weight were divided into three groups: intact, sham operated, and castrated. After 1 month they were inoculated (i.p.) with 4000 blood trypomastigotes. Parasitemia was lower in orchiectomized animals than in the intact and sham groups. Hormone replacement with decanoate testosterone raised the parasitemia of castrated animals to levels similar to those of their intact and sham counterparts. Antibody levels were monitored by complement-mediated lysis. The trypomastigote lysis percentage varied through the course of infection, according to hormonal status and number of parasites during the acute phase. The most significant differences were found on the 30th day after infection, when lytic antibodies of intact males were high compared to the orchiectomized and sham groups. Higher resistance with lower lysis indexes were observed after orchiectomy, compared to intact and sham males.