We have investigated the synthesis of C1q, C1r, C1s and C1-inhibitor in HepG2 cells, human umbilical vein endothelial cells (HUVEC), fibroblasts (skin and synovial membrane), chondrocytes and monocytes. C1q was only synthesised by monocytes, although the mRNAs for the C1qA and C1qC chains were expressed in HUVEC. C1r, C1s and C1-inhibitor were synthesised by all cell types. The secretion rates of C1r and C1s were approximately equimolar in fibroblasts and chondrocytes whereas the secretion rate for C1s exceeded that for C1r in the other cell types. Molar ratios of C1s to C1r were approximately 2:1 for HepG2 cells, 5:1 for monocytes and 10:1 for HUVEC. Stimulation with interferon-gamma resulted in increased expression of all four proteins. The C1s:C1r ratio did not alter in chondrocytes or fibroblasts, but approached unity in HepG2, monocytes and HUVEC, due to relatively greater stimulation of C1r gene expression.