Zebu cattle were infected with Trypanosoma congolense and T. vivax and changes in the levels of serum immunoglobulins were measured during infection and after treatment. Increases in the levels of IgM occurred within 14 days of infection coincidentally with the first appearance of trypanosomes in the peripheral blood. IgM levels rose to 2-10 times preinfection levels and remained high until the animals were treated with diminazene aceturate. Levels then declined to approximately pre-infection levels within 25 days. IgG levels showed little change. In one animal infected with T. congolense which died of trypanosomiasis, there was an initial increase in IgM levels but subsequently both IgM and IgG levels declined until the animal died. The neutralizing activity of whole serum, and serum IgM and IgG fractions from an ox infected with T. congolense were examined. High levels of neutralizing antibodies against trypanosomes isolated on the eighth day after infection were found in sera collected 12-29 days after infection. Most of the antibody activity was located in IgG fractions which showed a peak in neutralizing activity between day 17 and 22. IgG fractions of the sera showed only slight antibody activity.