H uman monocytes synthesized the third component of complement (C3) up to 5 wk in vitro. Evidence for net C3 synthesis was based on (a) incorporation of 14C-labeled amino acids into C3 protein, (b) indentity of the allotype of C3 produced in vitro with that of the doner's serum C3, even in the presence of carrier C3 protein of a different allotype; (c) correspondence of electrophoretic mobility, size, and subunit structure of C3 protein produced in vitro with serum C3; (d) inhibition of C3 production with cycloheximide. Monocytes from two unrelated C3-deficient patients were studied under conditions that supported C3 synthesis by normal monocytes. Serum from each of the patients contained less than 1% of the normal C3 concentration, buth their monocytes produced C3 at approximately equal to 25% of the normal rate when studied after 2 wk in vitro. The C3 produced in vitro by monocytes from one of the patients had the molecular weight of normal serum C3 and dissociated appropriately under reducing conditions. Monocytes from C3-deficient patients could not be distinguished from normals on the basis of morphology, rosetting with C3-coated erythrocytes, or rates of C2, and total protein synthesis.