HCC-1 is the only CC-chemokine known so far which circulates in nanomolar concentrations in human plasma. Its physiological function is not well defined. Posttranslational processing of HCC-1 was shown to modulate its biological properties. In this study several different processed forms of HCC-1 were isolated. Western blot analysis of human plasma extracts revealed a HCC-1 immunoreactive double band at 8-10 kDa indicating the presence of two distinct HCC-1 peptides. These peptides were isolated from a peptide library of human blood filtrate and represent predominantly HCC-1 (1-74) and glycosylated HCC-1 (1-74). Glycosylated HCC-1 exhibits a molecular mass of 9621 Da due to O-glycosylation at position 7 (Ser-7) with two N-acetylneuraminic acids and the disaccharide N-acetylgalactosamine galactose. Furthermore N-terminally truncated HCC-1 (3-74) and HCC-1 (4-74) were identified in the peptide library. In hemofiltrate approximately 3% of total HCC-1 represents HCC-1 (3-74) and approximately 1% represents HCC-1 (4-74) whereas the major products are nonglycosylated HCC-1 (1-74) and glycosylated HCC-1 (1-74). Our data imply that HCC-1 (1-74), HCC-1 (3-74), HCC-1 (4-74) and glycosylated HCC-1 (1-74) circulate in human blood. The N-terminal processing and modification of HCC-1 might be of importance in displaying its full biological activity.