Parasite-derived trans-sialidase (TS) activity was demonstrated in the serum and blood of Trypanosoma cruzi-infected mice. Serum TS activity levels correlated well with parasitemia in BALB/c and Swiss mice during the initial stages of the infection. However, in later stages the TS activity levels decreased despite increasing parasitemia. This coincided with the appearance of circulating TS antibodies. On the other hand, there was always a good correlation between TS activity and parasitemia in athymic nude mice. Sera from mice with high parasitemia and low TS activity inhibited TS activity in vitro. The inhibition was also observed with purified serum IgG, and it was absorbed by staphylococcal protein A, indicating that it was caused by anti-TS IgG antibodies. These antibodies inhibited the enzymatic activity of insolubilized TS, indicating that they act by interfering with the catalytic site rather than by aggregating the enzyme. The presence of inhibitory antibodies, however, did not prevent the progression of parasitemia in BALB/c mice.