Alleles of IL1A-889(C > T) and IL1A+4845(G > T) are in linkage disequilibrium. Interleukin 1 alpha (IL-1 alpha) is produced as a precursor protein and cleaved at positions 117-118 by calpain, generating a mature protein for export. IL1A+4845 affects amino acids expressed at position 114 and hence may modulate calpain-mediated cleavage. We sought evidence for this mechanism in intact cells. Blood leukocytes from heterozygous donors released more IL-1 alpha protein than cells from IL1A(1,1) donors, while release from IL1A(2,2) cells was variable. Genotype did not affect levels of IL-1 alpha mRNA, so differential cleavage of the precursor is a feasible mechanism. However, genotype also had no effect on inhibition of IL-1 alpha release by pretreatment with calpain inhibitors, and calpain inhibitors reduced IL-1 alpha and tumor necrosis factor alpha mRNA levels. Hence, calpain inhibitors probably affect inhibition of signal transduction pathway rather than cleavage of IL-1 alpha protein. As ratios of mu-calpain/calpastatin were lowest in heterozygous donors, genetically determined IL-1 alpha levels may modulate transcription of calpain and calpastatin. This could reduce the impact of IL1A genotype on IL-1 alpha secretion and amplify individual variation in levels generated in culture.