Properdin stabilizes the alternative complement pathway C3 convertase and is synthesized by monocytes and myelomonocytic cell lines. Hepatic production of properdin has never been documented, although most other complement components are synthesized by liver. Human liver-derived Hep G2 cells were examined for the ability to produce properdin by using the polymerase chain reaction (PCR). Amplified properdin message was detected by using Southern transfer and hybridization to a murine properdin cDNA probe. Sequencing of the PCR product revealed that the Hep G2 message was nearly identical to the cDNA sequence from U937 cells. Subsequently Hep G2 cultures were stimulated with interleukin (IL-6), 25 micrograms/ml, and culture supernatants were assayed for the presence of properdin by using dot blots. Properdin concentration increased over time, and we found no obvious difference between properdin production by IL-6-stimulated and unstimulated Hep G2 cells. Finally, alternative pathway decay assays confirmed the presence of functionally active properdin in the culture supernatant. Thus, functional properdin is a product of Hep G2 cells, suggesting that biosynthesis of properdin may occur in hepatocytes. Properdin synthesis was not augmented by IL-6, a finding that is consistent with previous observations that properdin is not an acute phase reactant.