Inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] 3-kinase catalyses the phosphorylation of Ins(1,4,5)P3 to Ins(1,3,4,5)P4. cDNAs encoding two isoenzymes of Ins(1,4,5)P3 3-kinase (3-kinases A and B) have been described previously. In the present study, we report the cloning of a full-length 2052bp cDNA encoding a third human isoenzyme of the Ins(1,4,5)P3 3-kinase family, referred to as isoform C. This novel enzyme has a calculated molecular mass of 75.207kDa and a Km for Ins(1,4,5)P3 of 6µM. Northern-blot analysis showed the presence of a transcript of approx. 3.9kb in various human tissues. Inositol trisphosphate 3-kinase C demonstrates enzymic activity when expressed in DH5αF′ bacteria or COS-7 cells. Calcium alone decreases the Ins(1,4,5)P3 3-kinase activity of the 3-kinase C isoenzyme in transfected COS-7 cells. This inhibitory effect is reversed in the presence of calmodulin. The recombinant bacterial 3-kinase C can be adsorbed on calmodulin–Sepharose in the presence of calcium. The present data show that Ins(1,4,5)P3 3-kinase C: (i) shares a conserved catalytic domain of about 275 amino acids with the two other mammalian isoforms, (ii) could be purified on a calmodulin–Sepharose column and (iii) could be distinguished from the A and B isoenzymes by the effects of calcium and of calmodulin.