The biosynthesis and secretion of the third component of complement (C3) has been studied with the macrophage cell line J774.2. C3 is initially synthesized as a single polypeptide chain precursor termed pro‐C3, of relative molecular weight (M_r) 170,000 that is post‐translationally modified by proteolytic cleavage into two polypeptides linked by disulphide bonds. The larger polypeptide, termed the alpha chain, has an M_r of 110,000–115,000, while the smaller beta chain has an M_r of 55,000–60,000. Pulse‐chase experiments indicate that the proteolytic processing of pro‐C3 occurs intracellularly, just prior to secretion. Unlike human C3, which has carbohydrate on both the alpha and beta chains, only the alpha chain of murine C3 is glycosylated. The carboxylic ionophores monensin and nigericin totally inhibit the proteolytic processing of pro‐C3 at a concentration of approximately in 10⁻⁶_M. This block on proteolytic processing was shown not to be mediated by changes in intracellular pH induced by the disruption of proton gradients. Rather, data from experiments using carboxylic ionophores and other perturbants of cellular physiology indicated that the enzyme(s) responsible for the proteolytic cleavage of pro‐C3 either reside in a cellular compartment with a neutral pH or are proteinases active over a relatively broad pH range.