The sensitivities of 3 strains of Trypanosoma congolense to isometamidium chloride (Samorin) were determined in mice and cattle, with the objective of evaluating sensitivity testing in mice as a means of predicting curative doses in cattle. Comparison of mouse effective dose 80% (ED80) or curative dose 80% (CD80) values with cattle minimum curative dose (MCD) values demonstrated a wide variation between trypanosome strains. Although a mouse test may give a broad indication of the sensitivity of a strain, it cannot be used to predict curative doses for cattle. It was concluded that care should be exercised in extrapolating the results of a mouse test to cattle.