Abstract Ablastin formed by animals in response to infections by rodent trypanosomes possesses the characteristics of an antibody. Partial resistance to Trypanosoma lewisi is demonstrable in animals previously injected with live Trypanosoma musculi . Antisera from T. musculi infected mice do not inhibit reproduction by T. lewisi bloodstream forms in vitro as efficiently as homologous antisera collected at similar times during infections, indicating a degree of specificity. Ablastin activity in antisera is not altered by treatment with 2-mercaptoethanol or by heatng at 60 °C for 3 hr. Sephadex G-200 gel filtration of early and late antisera from T. lewisi infected rats and assays with bloodstream forms cultured at 37 °C detect ablastin activity in the second major fraction eluted from the columns. Ablastin appears to be an antibody of the immunoglobulin G species. Immunosuppressant procedures utilized in studies of the host responses to rodent trypanosomes are reviewed and include: chemical agents, irradiation, splenectomy, reticuloendothelia blockade and thymectomy, and treatment with antilymphocyte and antithymocyte sera. Evaluation of the results of the application of these procedures to rodent parasite systems indicates ablastin is an antibody and supports the concept that the inhibition of trypanosome reproduction is separate and distinct from the first trypanocidal event responsible for the decreasing parasitemias observed during the infections. Recent studies concur and suggest that the first crisis in the infections is mediated by the combined actions of a thymus-dependent ablastin and a thymus-independent trypanocidal antibody.