To study the relation between serum complement levels and the chicken MHC (B) complex, complement haemolytic activity was measured in sera from hens from seven pure-bred B-typed White and one Brown Leghorn lines, and three ISA-Warren lines that had been divergently selected for antibody responses to sheep red blood cells (SRBC). Significant differences occurred in the serum haemolytic complement activities, both belonging to the classic (CPW) and the alternative (APW) pathways, among the 11 different haplotyped chicken lines. Hens with high CPW and high APW titres predominantly displayed the B2 or B21 haplotypes. Chickens with low CPW and APW were found in B14 and B15 haplotypes. Haplotype B14 appears to be different in complement levels when present into the pure-bred lines or into the ISA-Warren line selected for low antibody responses to SRBC. Otherwise, the presence of B21 in ISA-Warren line selected for high antibody responses to SRBC does not differ with the B21 in the inbred lines (except in the NL-line for CPW values). In general the haplotypes B2 and B21 are found in chicken lines with enhanced disease resistance, and the B15 haplotype has been connected with enhanced disease susceptibility. Our results suggest that levels of haemolytic complement activity, either from the classical or from the alternative pathways, may underlie part of the immunocompetence ascribed to the MHC (B) complex in chickens.