Complement levels and other indices of susceptibility to trypanosomosis, were determined in three major Nigerian zebu breeds, viz: White Fulani (WF), Red Bororo (RB) and Sokoto Gudali (SG). Three cattle each of WF, RB and SG breeds were intravenously infected with 1.5x106 Trypanosoma congolense and monitored until the PCV declined to ? 15% when they were treated with diminazene aceturate. Infection with trypanosomes caused significant (P0.05) reduction in the PCV, Hb concentration, RBC count, THC, AHC and C3 levels. The drop was lowest in WF and highest in SG. Parasitaemia was significantly (P0.05) higher in SG and RB than the WF counterpart. Of the three infected SG, two died at a mean of 28.0+14.1 days post-infection (pi), while the third was treated 38 days pi. Infected RB and WF were treated at a mean of 39.3+2.3 and 54.7+2.3 days pi respectively. Live weight loss was lowest in WF (4.8%), intermediate in RB (6.7%) and highest in SG (11.1%) at 35 days pi. The anaemia recorded in infected cattle was lowest in WF and highest in SG. There were significant (P0.05) decreases in complement activities which were lowest in WF and highest in SG.