Human peripheral lymphocytes, with the capacity to be stimulated by anti-.delta. exhibited greater PHA [phytohemagglutinin] responsiveness when cultured with anti-.delta. 1 or 12 h before PHA exposure than cells exposed to PHA alone. When these lymphocytes were preincubated with PHA 1 or 12 h before anti-.delta. activation, no augmentation of the PHA response was seen. Lymphocytes from donors with a high PHA response (low anti-.delta. activation) failed to show an enhancing effect on PHA responsiveness when pretreated with anti-.delta.. Anti-.mu. [goat] showed no synergistic effect on PHA responsiveness. Anti-.delta.-activated cells apparently enhance PHA responsiveness.