A method was developed for the assay of the CF-lectin factor in amniotic fluid using samples from fetuses identified by other means as having or not having cystic fibrosis (CF). The method involves protein removal from the amniotic fluid by filtration through Centricon-10 Amicon filters, admixture of filtrate with normal serum as a source of IgM, and incubation in 5% Dextran-Sulfate with mouse red blood cells to detect specific hemagglutination. Blind samples submitted from four different laboratories were used to evaluate the ability of the assay to differentiate CF fetuses from fetuses without CF. Storage of samples from one laboratory had been quite prolonged and was associated with poor correlations. Freezing and thawing of samples were shown to deactivate the CF-lectin factor. In three of the four studies, CF lectin was detected in 77% of pregnancies identified as CF at birth, prenatally through microvillar enzyme assay or through DNA analysis of terminated pregnancies. Eighty-three percent of samples identified as normal were confirmed as such by a lack of CF-lectin activity. These data suggest that CF fetuses produce, and are exposed to, the CF-lectin factor at least as early as the second trimester of gestation.