A number of macrophage-derived mediators have been implicated in the vascular changes of inflammation. We recently reported the isolation of a novel monokine, macrophage inflammatory protein 1 (MIP-1), which causes local inflammatory responses in vivo, and induces superoxide production by neutrophils in vitro. Purified native MIP-1 comprises two peptides with very similar physical characteristics. We report here the resolution of MIP-1 into component peptides by SDS-hydroxylapatite chromatography, and compare the NH2-terminal sequences of the two peptides, now referred to as MIP-1 alpha and MIP-1 beta. A synthetic oligonucleotide probe pool corresponding to the NH2-terminal amino acid sequence of MIP-1 beta was used to isolate a cDNA clone containing its coding sequence. The sequence codes for a 109 amino acid-long polypeptide, of which 69 amino acids correspond to the mature product. Comparison of this MIP-1 beta cDNA with our previously cloned MIP-1 alpha sequence reveals that the MIP-1 peptides, members of a growing family of potential inflammatory mediators, are distinct but highly homologous (58.9% sequence identity) products of different genes.