Monocytes have previously been shown both to augment and suppress human natural killer (NK) cell activity depending upon the conditions. An interleukin-1/interleukin-2 (IL-1/IL-2)-dependent mechanism has been shown to be involved in the augmentative effect. In the current study, the role of the method of monocyte isolation was evaluated. Monocytes isolated by Percoll gradient centrifugation were ineffective for modulating NK activity, but monocytes isolated by adherence from most donors exhibited increased augmentation with increased interval of adherence (up to 1 h). However, monocytes isolated by adherence from certain donors reproducibly exhibited increased suppression with increased interval of adherence. The observation of augmentation was correlated with an increase in the balance between IL-1 production and prostaglandin E (PGE) production by the monocytes. The roles of PGE2 and IL-1 were therefore examined by mixing these cytokines with enriched null lymphocyte preparations in the absence or presence of monocytes in the NK assay system. The participation of PGE2 was further examined using monocytes treated with indomethacin (10(-6) M), and the participation of monocyte-membrane-bound IL-1 was evaluated using monocytes fixed with 1% paraformaldehyde. The results revealed that PGE2 production is involved in the suppression of human NK activity by human monocytes, and the functional balance between IL-1 and PGE2 determines whether suppression or augmentation is observed. The data of this and previous studies are consistent with the suggestion that membrane-associated IL-1 is the important IL-1 moiety for the augmentation of human NK activity by monocytes.