Natural suppressor (NS) activity, the capacity of unprimed cells to suppress immunologic responses, is present in mouse, rabbit, and human bone marrow (BM). In this study we characterize NS activity in bone marrow cells of the rhesus monkey. Greatest NS activity was found in low-density cells (1.0600 to 1.0655 g/mL) obtained by density centrifugation on a discontinuous Percoll gradient. NS activity was further enriched when cells were separated by affinity for wheat germ agglutinin (WGA). Cells with high affinity to WGA demonstrated potent NS activity, whereas cells with low affinity to WGA had no NS activity. A significant relationship between NS activity and hematopoietic activity was demonstrated using in vitro assays of colony formation (CFU-GM and CFU-MIX). NS activity was not affected by treatment with monoclonal antibodies (MoAbs) to human Fc gamma receptors (Leu, 11a,b,c) or treatment with MoAbs to monkey natural killer cells. These findings extend our prior observations by showing that cells with NS activity, which apparently have WGA receptors, are present not only in murine BM but also in monkey bone marrow, and suggest that such cells may be involved in immunoregulation by primitive cells of BM.