The octapeptide sequence of peptide T is contained within the envelope of HIV and seems to mediate the viral binding to CD4 expressin cells, including monocytes. The biological activity of the alfa-aninobutyric acid pentapeptide derived from the C-terminal squence of peptide T in which the polar side chain of the threonine in position 4 is substituted by a hydrophilic group is measured by the monocyte chemotaxis assay. The chemotactic activity of human monocytes is assessed by determining the concentration at which the pentapeptide analog is maximally active and the effectiveness at that concentration, in comparison with peptide T and two shorter homologs, the pentapeptide and tetraapeptide. These experiments suggest that the synthetic analog is a potent chemotactic factor active at picomolar concentrations and that it competes with peptide T for the monocyte binding site.