Patterns of antibody production in relation to the presence or absence of trypanosomes and to drug treatment, were studied in 38 cattle. The animals were infected with one or more of the three groups of African pathogenic trypanosome. An indirect fluorescent antibody test was used. A stronger homologous antibody was noted. High levels of antibody were associated with the presence of circulating trypanosomes. Falling levels of antibody were associated with self-cure or chemotherapeutic treatment. Titre of homologous antibody was considered to be a valuable aid in ascertaining the stage of the disease, and may be useful in assessing the efficiency of chemotherapy in a herd.