P35 (L-ficolin) is a novel calcium-dependent lectin which may have a role in innate immunity. The concentration range of P35 in blood donor sera was found to be 1.1-12.8 (median 3.7) microg/ml, with most values below 6.0 microg/ml. Sera from women with recurrent miscarriage were found to have a generally similar distribution of concentrations, but 3 (4%) had values lower than the lower limit of normal defined by the blood donor sera. Cord blood samples had significantly lower levels of P35 (median 2.5 microg/ml) than adults, and cord P35 concentrations correlated with gestational age. During ethanol fractionation of plasma, P35 was enriched in Cohn Fraction III, but did not bind to mannan immobilised on 3M-Emphaze. P35 thus has some similarities to, and some differences from, mannan binding lectin.