The production of interleukin 1 (IL1), a pleiotropic monocyte-derived interleukin, can be induced in vitro by various stimuli. The present study shows that cytochalasins which inhibit actin filament polymerization in various cell types have no significant effect on IL1 production from human monocytic cells. On the contrary, microtubule disrupters such as colchicine, vinblastine, and vincristine dramatically potentiate (15- to 35-fold), in a dose-dependent fashion, cell-associated IL1 and to a lesser extent (2.5- to 7-fold) released IL1 in the myelomonocytic THP1 cell line and in adherent peripheral blood mononuclear cells. The enhancing effect of the drugs was blocked by actinomycin D and by cycloheximide and was accompanied by an increase of specific IL1 beta mRNA expression as measured by Northern blot analysis, thus indicating that these drugs act at a transcriptional or post-transcriptional IL1 gene expression level.