During an epidemic of group B meningococcal infection mean values obtained in 96 consecutively affected children showed a reduction in classical pathway function (CH50), normal alternate pathway function (AP50), C4 and factor B levels, and raised C3 levels. CH50, C3 and Factor B were however significantly lower in those children who had a rapid onset of illness, were in shock, had signs of septicaemia, had extensive skin purpura, or who died. The presence of detectable meningococcal antigen by Counter Immuno Electrophoresis (CIE) and laboratory evidence of Disseminated Intravascular Coagulation (DIC) also correlated with lower complement levels. The significant reduction in CH50 and Factor B in the more severely affected patients suggests that activation of both classical and alternate pathways occurs in group B meningococcal infection.