A modified cystine-free minimum essential medium has been used to address the question whether cysteine is an essential growth factor for bloodstream form trypanosomes or if its reducing power is sufficient to support parasite growth in axenic culture. Bloodstream-form trypanosomes, taken either from freshly isolated infected mouse blood or from logarithmically growing axenic cultures were transferred to a medium containing 20% dialysed foetal calf serum, 10 microM bathocuproine sulphonate and 250 microM cysteine. Growth curves of these cultures have been compared to those obtained in identical cultures containing no cysteine but cystine and reducing agents (beta-mercaptoethanol, monothioglycerol), or reducing agents alone. The results clearly show that cell growth was only obtained if cysteine was either directly added to the medium or was reduced from cystine by the action of reducing agents. However, neither reducing agents alone, nor D-cysteine, supported cell growth. Since cystine is not taken up by bloodstream form trypanosomes, and methionine is a regular constituent of the medium, we conclude from our results that cysteine is an essential growth factor for Trypanosoma brucei.