Abstract To determine the molecular basis of complement C3 deficiency in a Laotian kindred, the homozygous C3-deficient male propositus was studied. By ELISA, this individual's serum was determined to contain approximately 4 μg/ml C3 (0.3% of normal). In accord with this result, anti-C3 immunoprecipitation of [ 35S]methionine-labeled fibroblasts from this C3D individual revealed pro-C3 of normal size (180,000 M r), but in significantly reduced amounts (∼1% of normal fibroblasts), that was processed and secreted with normal-size α- and β-chains. In addition, C3-specific mRNA of normal size (5.2 kb) but in reduced quantity (∼1% of normal) was detected in this individual's fibroblasts by Northern analysis. The nucleotide sequence of the transcriptional initiation site, the promoter, and the IL-1β/IL-6 cis-regulatory elements of the C3-deficient gene are normal in this C3-deficient individual, indicating that the low C3 mRNA and protein levels are not caused by reduced C3 transcription that is the result of a cis-mutation. Moreover, cDNA sequencing studies revealed no defect in the C3-deficient mRNA, including the areas mutated in four previously characterized C3-deficient patients. These data indicate that (1) C3 protein deficiency in this Laotian patient results from reduced levels of C3-specific mRNA, (2) the small amount of expressed C3 protein is processed and secreted normally from the deficient cells, and (3) the molecular genetic defect(s), although not yet delineated, is different from those described in other C3-deficient individuals, thereby providing additional evidence for numerous mutations that cause inherited C3 deficiency in humans.