Coculture experiments between lymphocytes of a 17-year-old immunodeficient male, DL, and a group of normal subjects, assaying pokeweed mitogen (PWM)-stimulated Ig secretion as a measure of B-cell function, revealed immunoregulatory abnormalities. Initial studies disclosed that DL had corticosteroid-sensitive T suppressor (Ts) cells capable of blocking Ig secretion by both HLA-identical and HLA-nonidentical cells in coculture. Cocultures of DL's peripheral blood mononuclear cells could be induced to secrete Ig in large amounts after certain maneuvers-the most informative of which involved mixing prednisolone-treated DL mononuclear cells with any normal T lymphocytes except those from DL himself. When these same experimental manipulations were performed individually, i.e., prednisolone treatment of cultured DL cells to remove Ts activity, or mixing equal numbers of normal T cells with untreated DL mononuclear cells, Ig was not produced. The data indicated that the T-cell abnormalities in DL included an excess of Ts cells and a deficiency of T helper (Th) cells. When repeat studies were performed later in the clinical course, during which interval a number of clinical interventions were attempted, it was found that the patient's cells were no longer corticosteroid sensitive and, further, they suppressed only HLA-identical cells.