Patients with active Hodgkin's disease (HD) often demonstrate an impaired T-cell proliferative response to phytohemagglutinin (PHA). The present study examined if interleukin regulation of the PHA response was defective in HD. The Hodgkin's PHA response was impaired at all concentrations of PHA utilized. Indomethacin increased the proliferative response but did not bring it to control levels. Stimulation of the cells with both PHA and irradiated Ia+ B cells normalized proliferation despite identical PGE2 concentrations as in the PHA alone cultures. Hodgkin's monocytes produced normal amounts of interleukin 1 (IL-1). Interleukin 2 (IL-2) production by Hodgkin's T cells was decreased in the PHA stimulated cultures, but was normal in the PHA and Ia+ cell stimulated cultures. In response to PHA stimulation alone, Hodgkin's T cells expressed less IL-2 receptor than control cells. The data suggest the diminished PHA response in HD is due to impaired IL-2 production resulting in diminished IL-2 receptor expression. However, when an Ia+ cell source is added to PHA as an additional stimulator, both TCGF production and proliferation are normalized. Monocytes serve to modulate the magnitude of the PHA response through production of both interleukin 1 and PGE2. However, in the presence of sufficient IL-2 production the influence of monocytes is minimized.