C7 M/N typing, the determination of the allotypes of the recently described C7 M/N protein polymorphism, was conducted on serum samples from donors and recipients of 100 liver transplantations to determine whether the liver is the predominant site of in vivo synthesis of human complement protein C7. Twenty-one cases were informative as the recipient was transplanted with the liver obtained from a donor with a different C7 M/N allotype. The determination of the C7 levels and phenotypes of up to 10 post-transplantation (p.t.) samples revealed that there was at most only a 50% contribution of the transplanted liver toward the C7 M/N allotype at 2 to 3 wk p.t.; that influence decreased with time and was approximately 10% in the samples obtained later than 6 wk after transplantation. C7 is thus the only terminal complement component not predominantly synthesized by hepatocytes, which is compatible with the observation that C7 is not an acute phase reactant. The transient contribution of the donor phenotype appears to be attributable to cells of the mononuclear phagocyte lineage, Kupffer cells in particular that are replaced by cells of recipient origin. Various cells of that lineage that are known to synthesize C7 in vitro, therefore, contribute more toward the C7 concentration than previously anticipated. Enhanced local C7 synthesis at the site of inflammation might add further to the basic C7 level especially because C7 is often the limiting factor for terminal complement complex generation.