Interleukin (IL)-1alpha and IL-1beta are encoded by two separate genes, but both function as comitogens for lymphocyte activation. In this study, we observed K-562 cells to express constitutively mRNA for IL-1alpha, although IL-1alpha was not detected in the growth-conditioned medium (GCM). However, IL-1beta mRNA was not expressed unless the cells had been treated with phorbol myristate acetate (PMA). Both IL-1alpha and IL-1beta were detected in the GCM after the cells had been cultured with PMA, suggesting that IL-1 elaboration required PMA treatment. The K-562 cells treated with PMA differentiated to the myeloblastic stage, as observed by nuclear morphologic properties by electron microscopy. PMA treatment induced de novo expression of CD61 or gpIIIa, a marker associated with megakaryoblasts. These results showed that although K-562 cells constitutively expressed IL-1alpha mRNA, PMA treatment was required for secretion. On the other hand, both the expression and secretion of IL-1beta required treatment with PMA. This study showed that K-562 cells treated with PMA differentiated to the myeloblastic stage and expressed and secreted IL-1alpha and IL-1beta.