Failure to opsonise bakers' yeast (Saccharomyces cerevisiae) is a defect found in 5-7% of the general population. In this study, the presence of the defect was linked with low levels of mannan-binding protein (MBP), a calcium-dependent serum lectin. Purified MBP corrected the defect in a dose-dependent way in an in-vitro assay measuring the deposition of complement moieties on a mannan-coated surface. There was a highly significant correlation between the serum MBP level and the generation of C3b opsonins in a population of healthy blood donors. The median MBP level of ten children previously shown to have the functional opsonic defect was 4.9 micrograms/l (range 2.5-35.0 micrograms/l) compared with 143 micrograms/l (range 2.5-880 micrograms/l) for a paediatric control group.