Crude preparations of kinetoplast vesicles were used to investigate the respiratory chain components in L. tropica promastigotes. In difference spectra from enzymically and chemically reduced preparations, cytochrome b was the predominant component. By utilizing special assays designed to minimize the influence of cytochrome b on difference spectra, cytochromes a, a3 and c555 were demonstrated. Difference spectra from chemically reduced preparations indicated that pyridine nucleotides (NADH) and flavoproteins were also part of the respiratory chain. The presence of these components and their response to respiratory inhibitors and ascorbate provide evidence for the presence of a typical trypanosomatid respiratory chain in L. tropica promastigotes.