Hemofiltrate C-C chemokine (HCC)-1 is a recently described monocyte chemoattractant. CCL14 (also known as HCC-1) belongs to the CC chemokine family. Its mature propeptide is a low-affinity agonist of CCR1 that is converted to a high-affinity agonist of CCR1 and CCR5 on proteolytic processing by serine proteases. Determination of the amino acid sequence of HCC-1 revealed four cysteine residues in positions characteristic of the C-C chemokine family, and comparison with the sequences of other chemokines revealed that HCC-1 was most homologous to MIP-1a. However, several functional properties of HCC-1 were atypical of chemokines. Unlike other chemokines, HCC-1 was expressed constitutively in a number of tissues and was present at high concentrations in normal human plasma. In addition, HCC-1 was reported not to be chemotactic for leukocytes.