The purpose of this study is to show the effect of caffeine and theophylline on the activities of aspartate aminotransferase AST and alanine aminotransferase ALT, in the human sera. Serum AST and ALT were activated by caffeine, while inhibiting by theophylline, this effect increased with increasing the concentration of theophylline and caffeine. Kinetic properties of AST and ALT activities revealed by theophylline and that non-competitive inhibition type, and non-competitive activation by caffeine.