Previous studies have shown that, after receptor-mediated endocytosis, interleukin-1 alpha (IL1 alpha) and interleukin-1 beta (IL1 beta) are translocated to the nucleus, where they appear to accumulate. It has been suggested that nuclear translocation may be involved in the biological responsiveness of target cells to IL1 stimulation. The human IL1 beta molecule contains a seven-amino-acid sequence (-Pro208-Lys-Lys-Lys-Met-Glu-Lys-) that shows some sequence identity with the nuclear localization sequence of the simian-virus-40 large T-antigen. The effects of point mutations within this putative nuclear localization sequence on IL1 beta binding, receptor-mediated endocytosis and biological activity have been characterized. Mutants M49 (Lys210----Ala), M50 (Lys211----Ala) and M51 (Pro208----Ala) all retained the ability to bind to the IL1 receptor, albeit with lower affinity than the wild-type molecules. However, mutants M49, M50 and M51 showed greater biological potency than wild-type IL1 alpha or IL1 beta, as measured by the induction of IL2 secretion. However, receptor-mediated endocytosis and nuclear accumulation of M50 were comparable with those in the wild-type. These observations suggest that the putative nuclear localization sequence may play an important role in the generation of biological responses to IL1 stimulation, even though it may not influence internalization of the ligand.