Monoclonal antibodies (MAbs) against two non-cross-reacting antigens of human IL-1 beta (Vhp20 and BRhC3) and human TNF alpha (B154.2 and B154.7) were applied to identify cytokine-containing cells in tissue sections and in cell suspensions. IL-1 beta- or TNF alpha-positive cells were not present in immunostained cytocentrifuge smears prepared from freshly isolated peripheral blood leukocytes, spleen, and lymph node cells. After 18 hours of culture with bacterial endotoxin (LPS), 80% to 90% of blood monocytes, 30% of spleen macrophages, and 2% to 28% of lymph node macrophages were strongly positive for IL-1 beta with either of the MAbs. Furthermore, 25% to 35% of blood monocytes and 6% to 60% of lymph node macrophages were stained for TNF alpha. Cells positive for IL-1 beta or TNF alpha were extremely rare in sections of normal thymus, spleen, and lymph nodes. Immunoreactivity for IL-1 beta or TNF alpha was frequently observed in sections of granulomatous lymphadenitis (N = 11). IL-1 beta or TNF alpha staining was confined to the epithelioid macrophages forming the granuloma, and the intensity of TNF alpha reactivity was generally stronger. The high frequency of cytokine-containing cells in this pathologic condition was confirmed in a cell suspension study showing that 20% of epithelioid macrophages were weakly positive for IL-1 beta and 80% were strongly positive for TNF alpha. The presence of cytokine-containing cells was investigated in cryostat sections of several nonlymphoid organs with normal histologic appearance. IL-1 beta reactivity was not observed in any of the tissues. TNF alpha reactivity was frequently demonstrated in isolated macrophages embedded in the interstitial connective tissue.