Both theophylline and halothane inhibited transformation of human lymphocytes by phytohemagglutinin (PHA). Theophylline did not augment the inhibitory action of halothane and depressed PHA transformation of halothane-treated cells to the same extent as that of air-treated cells. Halothane- and air-treated lymphocytes, prior to PHA addition, had the same content of cyclic AMP. The addition of PHA to these cultures raised cyclic-AMP concentrations to the same extent in halothane- and air-treated lymphocytes. Halothane action on PHA-stimulated lymphocytes appears not to involve changes in cyclic nucleotide metabolism.