Administration of 1mM sodium butyrate or N6,2'-O-dibutyryladenosine 3':5'-cyclic monophosphate (dbcAMP) inhibits the growth activity of U937 human monoblastoid cells by blocking them at the G1 or at the G1 + G2 phases of the cell cycle, respectively. Both agents induce the differentiation of U937 cells, as proved by the increased expression of the maturation-associated CD11b antigen and by the increased capacity to reduce nitroblue tetrazolium. RNA blot assays indicate that butyrate and dbcAMP decrease the expression of ornithine decarboxylase and c-myc genes, and stimulate the expression of the vimentin gene. However, while dbcAMP induces c-fos mRNA accumulation, butyrate did not affect the expression of this proto-oncogene.