Serum concentrations of circulating immune complexes (CIC), complement factors (Factor B, C4, C8) and complement activities (CH50 and AH50) were determined in Nigerian school children having urinary schistosomiasis with or without symptomatic malaria by polyethylene glycol precipitation method, single radial immunodiffusion and total haemolytic activities respectively. One hundred and forty-seven children were recruited from St. John's Primary School, Mokola, Ibadan, Nigeria.RP ovale only, mixed infection of P. ovale with P. falciparum or mixed infection of P. malariae with P. falciparum were found in subjects with asymptomatic malaria without urinary schistosomiasis (M-USS) but P. malariae or P. falciparum was found in subjects with co-infection of urinary schistosomiasis and asymptomatic malaria (M + USS). Mean value of C4 concentration was significantly reduced in M - USS subjects or subjects having both USS and asymptomatic malaria (M + USS) compared with non-infected controls. Serum concentration of Factor B(FB) was significantly reduced while AH50 was significantly increased in urinary schisosomiasis subjects without malaria (USS-M) compared with M-USS subjects or the controls. These observations implied that complement system in USS-M subjects is activated predominantly via alternative pathway (APW) while complement system is activated via classical pathway (CPW) in M-USS or M+USS subjects. The switch of complement activation pathway from alternative type in USS-M subjects to classical type in M-USS subjects may explain the lower malaria parasite densities often found in children harbouring Schistosoma haematobium parasites.