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Biosynthetic radiolabeling studies demonstrate that human keratinocytes and A-431 cells, a human epidermoid carcinoma cell line, synthesize and secrete factor B as a monomeric 105-kD protein. Epithelial cell-derived factor B comigrates in SDS-PAGE with that produced by HepG2 cells, a human hepatoma cell line traditionally employed in studies of complement component biosynthesis. Comparative pulse-chase studies in A-431 and HepG2 cells show that this alternative pathway complement component is produced as co-migrating 100-kD intracellular proteins that are processed in both cell types to 105-kD extracellular factor B. Quantitatively, immunoprecipitable factor B accounts for 0.05% of radiolabeled proteins in A-431 cell culture media. Treatment of biosynthetically radiolabeled A-431 cell culture media with cobra venom factor and factor D for 60 min at 37 degrees C produces the specific factor B cleavage products Ba and Bb. These fragments are not identifiable in control culture media subjected to similar treatment in the absence of alternative pathway activators. Northern blot analysis of total cellular RNA from human keratinocytes, A-431 cells, and HepG2 cells reveals qualitative identity of a 2.8-kb factor B mRNA species in these three cell types. The relative level of factor B mRNA expression in these cells parallels their level of factor B protein synthesis (i.e., HepG2 cells greater than A-431 cells greater than human keratinocytes). Epithelial cell-derived factor B may play an important role in local inflammatory reactions and also directly interact with epithelial cell derived C3--a key classical and alternative pathway complement component recently shown to be produced by human keratinocytes.