We have used synthetic peptides coupled to KLH to raise high titer antisera to human IL-1 beta, and in the present report show the usefulness of these sera for immunocytochemical analyses of IL-1 production. Using indirect immunofluorescence, we have been able to specifically identify IL-1 within human monocytes and to monitor its accumulation with time. After indirect immunofluorescent staining of LPS- and PHA-stimulated mononuclear cell cultures, intense cytoplasmic fluorescence was observed in 93% of the monocytes, but not in lymphocytes or platelets present in the same preparation. Unstimulated monocytes did not contain immunocytochemically detectable IL-1. When put into culture, however, some of the otherwise unstimulated monocytes subsequently showed a transient accumulation of intracellular IL-1. Monocytes cultured in the presence of LPS and PHA exhibited detectable fluorescence after 2.5 h, and the fluorescent intensity of these cells continued to increase over the course of 21 h. Fluorescent staining was abolished by preincubation of the sera with relevant but not irrelevant peptide, and while preimmune or anti-KLH serum produced no staining, antisera against either the amino terminus or an internal region of IL-1 beta produced identical staining patterns. Immunoblot analyses of lysates from stimulated monocytes showed that the antisera against IL-1 recognize a single intracellular species with an apparent molecular weight (33 kD) similar to that predicted for IL-1 precursor from the nucleotide sequence of IL-1 cDNA. The ability to specifically identify and immunocytochemically localize IL-1 within producing cells should prove extremely useful for studying the in situ production of IL-1 in immune-based and inflammatory diseases.