Results obtained in experiments testing the efficacy of anti-procyclic-form rabbit sera on the development of homologous and heterologous stocks of Trypanosoma brucei brucei in Glossina morsitans morsitans indicated that this development was affected little, or not at all, by such sera. The absence of effect of anti-procyclic stage antibodies can be explained by the failure to detect by either direct or indirect fluorescent antibody methods the presence of antibodies acquired in vivo by either the midgut procyclic forms or by uncoated salivary gland forms.