AbstractHuman adherent monocytes were incubated with interferon (IFN) preparations, then washed and stimulated with endotoxins. The interleukin (IL1) activity in the supernatants of IFN‐treated monocyte cultures was found to be increased as compared to control (not pretreated) cultures. This phenomenon was observed whether α or β IFN was used, and was completely abolished by antiserum to the relevant IFN. IL1 secretion of IFN‐treated monocyte cultures could be triggered by suboptimal dosis of endotoxins, unable to induce any IL1 activity in control medium‐treated cultures. IFN was highly efficient in this system, since very low concentrations (of the order of 2 units) were effective. Since IFN alone, without endotoxin stimulation, was unable to induce IL1 secretion, the results indicate that IFN is able to activate monocytes, by inducing a potential secretory capacity rather than an ongoing IL1 secretion function. The ability of IFN to enhance the IL1 secretory potential of human monocytes may be relevant to the known immunoenhancing effects of IFN preparations and to the pyrogenic effects of IFN administration in p