Opsonisation of heat-killed baker's yeast, functional activity of the total alternative pathway of complement, and factor B detected functionally and immunochemically were significantly reduced in 72 children with sickle cell disease compared with 40 age-matched black control children. There was significant correlation between functional activity of the total alternative pathway and functionally measured factor B, but not between factor B measured functionally and immunochemically. The opsonisation defect could be corrected in vitro by normal serum, and factor B-depleted serum, and was qualitatively similar to that seen in patients with primary yeast opsonisation deficiency. Serial studies showed that these serum defects were persistent. Reduction in the activity of components of the alternative pathway of complement and opsonisation was found in 4 patients who had recovered from pneumococcal meningitis and in one who developed osteomyelitis. Defects of yeast opsonisation and complement which are common in patients with sickle cell disease, may partly explain the children's increased susceptibility to infection, and might help to identify individuals especially at risk.