The sensitivity of Trypanosoma congolense isolates to diminazene and isometamidium was determined using an incorporation assay based on the uptake of [3H]-hypoxanthine in the presence of serial drug dilutions. The bloodstream forms of the different isolates exhibited variation in their sensitivity to the drugs that correlated well with the in vivo drug response. For diminazene, the sensitivity of the most sensitive population was 40 times that of the least sensitive population. For isometamidium, the IC50 values (the drug concentrations that decreased radiolabel incorporation by 50%) lay in a similar range, except for those found for two isolates from lions, which were 10(3)-10(4) times more sensitive than the isolates from cattle. The sensitivity of procyclic forms differed markedly from that of the bloodstream stages. Therefore, it must be concluded that the procyclic stage does not reflect the sensitivity of the bloodstream forms of T. congolense and that the former should not be used for determinations of in vitro drug sensitivity.