Trypanosoma vivax produced a progressive macrocytic normochromic anaemia in sheep during the acute phase of infection. Reticulocytes were absent from the blood of healthy sheep and of sheep with T. vivax-induced anaemia. However, anaemia induced artificially (AHA) in sheep by in vitro heat treatment of red cells, which was comparable in classification and degree to the anaemia of T. vivax infection, produced a reticulocytosis of 1.5 +/- 1.0 per cent. When plasma from the anaemic blood of sheep infected with T. vivax for two and four weeks was inoculated subcutaneously into mice, the reticulocyte response was similar to that of mice that received no sheep plasma and inferior to that elicited by normal sheep plasma. The anaemic plasma from sheep infected with T. vivax for three weeks induced a moderate reticulocyte response in mice which was, however, less intense than that induced by plasma from sheep with artificially induced anaemia of comparable intensity. These results indicate that, although the macrocytosis suggests that T. vivax-induced anaemia in sheep is slightly responsive, this response is suboptimal since reticulocytes were lacking in the blood of the sheep and their plasma was weakly erythrogenic in mice. This contrasts with the mild reticulocytosis in sheep with AHA of the same intensity and classification, whose plasma also stimulated considerable erythropoiesis in mice. The poor stimulation by plasma from T. vivax-infected sheep at two and four weeks post-infection suggests subnormal erythropoietin at these periods of infection.