Monocyte infiltration occurs early in the course of inflammation and is a prerequisite for optimal repair of tissue damage. In this study, human recombinant growth hormone was shown to be a potent chemoattractant for human monocytes, inducing migration at picomolar concentrations of recombinant human growth hormone. Chemotaxis of monocytes was measured in vitro by a modified Boyden chamber assay using nitrocellulose micropore filters and measuring microscopically the migration depth of the leading front of monocytes. Somatostatin, which inhibits the release of growth hormone, and its long-acting analogue, octreotide, also stimulated chemotaxis of monocytes; however, the effective peptide concentration was in the micromolar range. When tested for chemotaxis in combination or in experiments using pretreatment with somatostatin and washing of treated cells, somatostatin significantly antagonized the chemotactic responses of monocytes to growth hormone. The inhibitory effect on growth hormone- stimulated chemotaxis was dose dependent and occurred at concentrations severalfold lower than the chemotactically active concentration of somatostatin. Combinations of growth hormone with interferon or substance P also deactivated the chemotactic responses. These observations suggest that human growth hormone may have a regulatory role in monocyte chemotaxis.