Kynurenine aminotransferase and glutamine transaminase K and L activities were assessed in order to identify a physiological role for human hepatic cytosolic C-S lyase (CSL), as the C-C lyase kynureninase, which copurifies with rat hepatic CSL, has been shown not to be human hepatic CSL. Kynurenic acid production supports an aminotransferase role for this CSL.