Low serum mannose-binding protein (MBP) concentrations are associated with a common opsonic defect. Sequence analysis of the MBP gene in three children with recurrent infections, the opsonic defect, and low serum MBP concentrations showed a point mutation at base 230 of exon 1 causing a change of codon 54 from GGC to GAC. The replacement of glycine with an aspartic acid residue disrupts the fifth Gly-Xaa-Yaa repeat in the collagen-like domain of each 32 kD MBP peptide chain and probably prevents the formation of the normal triple helix. Study of sixteen members of the three families showed autosomal dominant coinheritance of the mutation and low serum MBP concentrations.