AbstractRecently, we have shown that the collagen‐like, Fc‐recognizing subcomponent C1q of the first complement component is synthesized by human, guinea pig and mouse peritoneal macrophages. To test whether macrophages may contribute to the serum pool of C1q, C1q was purified from guinea pig serum and from guinea pig peritoneal macrophage supernatants and compared for similarities. Both molecules had a similar sedimentation rate (macrophage C1q: 11.3S, serum C1q: 11.2S) and showed on sodium dodecyl sulfate‐polyacrylamide gel electrophoresis under reducing conditions three identical bands with molecular weights of Mr 29 000, Mr 27 000 and Mr 23 000 for the A, B and C chains, respectively. Both C1q molecules migrated by immunoelec‐ trophoresis in the gamma region and, in Ouchterlony analysis, showed complete antigenic identity with rabbit anti‐serum C1q. These experiments demonstrate the antigenic and protein chemical similarities between serum C1q and C1q secreted by macrophages supporting the idea that macrophages have to be considered as one potential source of serum C1q. Furthermore, macrophage‐derived C1q may be of importance in the local microenvironment at an inflammatory site involving macrophages.