The major interleukin 1 beta (IL-1 beta) species from human plantar stratum corneum was purified and found to have an N-terminal amino acid sequence homologous to a stretch of the human IL-1 beta precursor, starting with His115. Whereas SDS-polyacrylamide gel electrophoresis followed by immunoblotting revealed only one component in plantar stratum corneum with IL-1 beta-like immunoreactivity, and with an apparent molecular mass around 18 kDa, isoelectric focusing under non-denaturing conditions showed one major component with isoelectric point around 6.1 and two minor components isoelectric at pH 6.3 and 6.9, respectively. Digestion of recombinant human IL-1 beta precursor with chymotrypsin, producing a C-terminal fragment with N-terminal Yal114, yielded a component with IL-1 beta-like immunoreactivity isoelectric at pH 6.3. Recombinant bacterial variants of human IL-1 beta with N-terminal amino acids corresponding to Val114, His115 and Ala117 were isoelectric at pH 6.3, 6.1 and 6.9, respectively. Cloning and subsequent nucleotide sequencing of IL-1 beta precursor cDNA from a human keratinocyte line showed total identify with the sequence previously published for the human monocyte IL-1 beta precursor. The authors conclude that the IL-1 beta species present in plantar stratum corneum have isoelectric points determined by their respective amino acid sequences, and that there is a mechanism for IL-1 beta activation in human epidermis not involving interleukin 1 beta convertase.