It was shown that irradiation (650 rad) of 7 inbred strains of mice did not block the ability of Trypanosoma brucei rhodesiense to transform from the long slender (LS) to the short stumpy (SS) form or alter the plateau in parasitemia. In addition, it was observed that significant differences in parasitemia levels, in the rate of transformation from the LS to the SS form, as well as in the survival times occurred between the irradiated C3HeB/FeJ and several of the other strains. These differences in the nonspecific ability to control parasitemia appeared to be characteristic for each inbred strain of mice. The resistant strains generally had lower parasitemia than the susceptible strains. However, it was also shown that there is not a one-to-one correlation between the innate ability of a mouse strain to control its initial parasitemia, and the strain's ability to clear the parasitemia or increase its survival time. It was therefore concluded that the hypothesis which states that the ability of an animal to increase nonspecifically the rate of transformation, and therefore to lower the parasitemia, allowing intact animals to respond immunologically and survive longer is either incorrect or incomplete. The results further show that the ability of mice to clear their initial parasitemia by an antibody response is not necessarily correlated with their survival time. Therefore, this study suggests that factors other than an antibody response and the nonspecific control of parasitemia are important in resistance.