Antibody ligation of CD164 on primitive CD34⁺CD38⁻ HPCs suppresses cytokine-induced recruitment of cellular division. Single CD34⁺CD38⁻ cells were deposited on wells of a Terasaki plate coated with equal concentrations of either the anti-CD164 antibody 103.B2 (▴) or isotype-matched binding (□) and nonbinding control antibodies (○). Cellular proliferation was stimulated with IL-3, IL-6, G-CSF, and SCF (36GS). Cultures were monitored at regular intervals over 14 days to assess cell growth. Culture of CD34⁺CD38⁻ cells on either of the two control antibodies led to the recruitment of approximately 40% of the population into division, whereas the CD164 antibody resulted in a specific and significant (P < .05) reduction by approximately 50% at each of the time points measured in the proportion of CD34⁺CD38⁻ cells recruited into division under these assay conditions. Thus antibody ligation of CD164 on CD34⁺CD38⁻ cells suppresses cytokine-induced recruitment of the cells into division.