Early during the course of Trypanosoma congolense infection in cattle decreases in PCV occurred and coincided with increases in both MCV and MCH. The indices reached highest levels between eight and 12 weeks post-infection. By week 20 of infection MCV and MCH had decreased to pre-infection levels even though a substantial anemia persisted. Serum iron levels were elevated at eight weeks postinfection. (Infected 271 mg dl-1 v. control 140 mg dl-1) but decreased to low levels in infected animals by Week 28 (63 mg dl-1). At eight weeks post-infection PITR was elevated in infected animals (infected 2.6 mg dl-1 day-1 v. control 0.82 mg dl-1 day-1). By Week 28, however, PITR had fallen in infected animals to 0.62 mg dl-1 day-1 indicating a severe dyshaemopoesis since PCV levels averaged only 22%. Cattle which were treated with Berenil or that self-cured the infection had normal iron parameters when tested 61 weeks post-infection. Cattle treated early during the course of infection showed a significantly greater PCV response that those treated later during infection.