The third component of complement (C3) synthesized by human monocyte-derived macrophages has been shown to have the same size and sub-unit structure as serum C3, but haemolytic activity has not been demonstrated. Human monocyte-derived macrophages were cultured from days 4 to 7 in medium without serum, and the conditioned medium was dialysed to remove inhibitors of the C3 assay and concentrated to enhance detection of low amounts of C3. Using these techniques C3 activity was detected routinely. The amount of C3 was 3.4 x 10 (7) effective C3 molecules/ml of concentrated tissue culture medium (range 1.0-7.5 x 10 (7)), and the number of C3 molecules synthesized by each cell was 4.4 x 10 (5), assuming that each cell synthesized C3. The specific activity of the C3 synthesized by the monocytes was the same as the specific activity of C3 that had been purified from serum and then incubated with the cells and processed in the same manner as the monocyte media. Synthesis as the basis for the presence of the C3 activity in the medium was indicated by an inhibition of production of the C3 activity of 66+/-16% by cycloheximide, 2 micrograms/ml. Thus, human blood monocytes that migrate into areas of inflammation can mature into cells capable of producing C3 which can participate in the complement sequence and thus potentiate inflammation.