Five alternatively spliced mRNA isoforms of human caspase-1 have been identified previously and we report here the cloning of a new isoform, named CASP1 zeta (zeta), from human ovarian surface epithelial cell cDNA. The new isoform zeta is identical to the alpha isoform but missing 79 nucleotides in the coding region of the prodomain of procaspase-1. Analysis of the cDNA sequence of the zeta isoform revealed an ORF of a shorter protein missing the 39 amino acids at the amino terminal of procaspase-1alpha, which comprises the important caspase activating recruitment domain (CARD), which is required for interactions between caspases and other proteins. Secondary structure analysis of procaspase-1 CARD predicted the truncation of the alpha1, the alpha2, and part of the alpha3 helix in the zeta isoform in comparison to the full-length alpha isoform. The new zeta isoform was expressed in many, but not all, adult human tissues by RT-PCR. In HEK293 cells, transient overexpression of wild-type caspase-1zeta induced apoptosis to levels similar to those of caspase-1alpha. However, mutational change at the caspase-1 active center of the Cys 246 of caspase-1zeta, as well as Cys 285 of caspase-1alpha, completely abolished their apoptotic activity. Our findings suggest that caspase-1zeta is a widespread, new proapoptotic isoform of caspase-1. They also demonstrate that the first 39 amino acids of the N-terminal of the CARD in procaspase-1 are not required for its apoptotic activity.