The interaction between antilymphocyte globulin (ALG) and phytohemagglutinin (PHA) in human lymphocyte culture has been studied. Treatment of the cells with rabbit anti‐human lymphocyte globulin (RAHLG) was found to inhibit their response to PHA. This combined ALG‐PHA effect in the initial stages of lymphocyte stimulation may be explained by a model of drug‐receptor interaction which shows that PHA is inhibited competitively by