Alpha-Macroglobulins (AM) are protease inhibitors with important roles in inflammation and in immunomodulation that behave as acute-phase proteins in many experimental models. In the present work the levels of AM in the plasma of outbred Swiss albino mice acutely infected with Trypanosoma cruzi were studied. The results showed that increased levels of AM were present in the majority of the infected mice and that AM levels increased independently of the rise in parasitaemia. There was a high degree of heterogeneity in the intensity of the modulation of AM levels as well as in the kinetics of AM synthesis. This heterogeneity was related neither with the intensity of infection nor with the sex of the host. No correlation between AM levels and survival to the acute phase could be observed in the outbred mice. The consequence of such a heterogeneity is unclear, although AM as immunoregulatory molecules could play a role in the development of the symptoms of the chronic phase of Chagas' disease.