An in vitro assay to distinguish drug-resistant from drug-sensitive populations of Trypanosoma congolense has been developed. The incorporation of radiolabelled hypoxanthine by procyclic trypanosomes in vitro was measured after 48 h exposure to different concentrations of trypanocides. In the presence of either isometamidium chloride (Samorin) or diminazene aceturate (Berenil), the ability of procyclics of a drug-sensitive stock (TREU 1627) to incorporate hypoxanthine at 28 degrees C was impaired to a much greater extent than that of procyclics of a drug-resistant stock (TREU 1467), when compared with control organisms grown in the absence of drugs. Serum from a rabbit given 1 mg/kg Samorin also inhibited incorporation of radiolabel in TREU 1627 procyclics more severely than in TREU 1467 procyclics, although the difference between stocks was not substantial. When used with cultured blood-stream forms maintained at 35 degrees C, the assay could distinguish the stocks in the presence of Samorin, but no difference was detected between the populations in their incorporation of hypoxanthine after exposure to Berenil.