Fresh human plasma was treated with proteinase inhibitors and passed through an immunoadsorbent column of Sepharose anti-C3 globulin. The insolubilized C3 was eluted with 5 M guanidine and, after extensive dialysis, was reduced with 0.2 M 2-mercaptoethanol and electrophoresed on SDS-polyacrylamide gels. The bulk of the eluted C3 dissociated into two major protein bands, m.w. 125,000 and 75,000 corresponding to the alpha- and beta-chains of C3. In addition, a nonreducible single polypeptide chain (SPC) with a m.w. value of 197,000 +/- 2,000 similar to the apparent m.w. of unreduced C3 was consistently present. SPC has been purified by elution from SDS (SPC) and found to remain a single polypeptide chain upon re-electrophoresis on SDS gels in the presence of 0.2 M 2-mercaptoethanol. The purified SPC reacted with antisera to denatured C3, C3alpha, and C3beta chains. Additionally, antisera to SPC, also reacted with denatured C3, C3alpha-, and C3beta-chains, revealed a reaction identity between SPC and C3, and detected partial identity between SPC and C3alpha- as well as C3beta-chains. This suggested that SPC and C3 are antigenically related. The amino acid compositions and tryptic peptide maps of SPC and C3 were similar. Based on these findings, it is suggested that SPC must represent a single-chain precursor C3 (pro-C3) in plasma that escaped post-synthetic proteolytic cleavage into a two-subunit chain C3 molecule.