The aim of this study was to examine the cytokine production and cytokine responsiveness of the first T-cell receptor (TcR) positive cells that appear in the murine fetal thymus, namely TcR V gamma 3 cells. It is shown that IL-2-cultured fetal TcR V gamma 3 thymocytes were capable of producing IL-3, GM-CSF, TNF-alpha and IFN-gamma upon TcR triggering. IL-2, IL-4, IL-5 and IL-6 could not be detected. With regard to cytokine responsiveness, TcR V gamma 3 cells proliferated to a high extent when high concentrations of rIL-2 were added. rIL-4 or rIL-7 alone, but not rIL-1 alone, were capable of inducing a modest proliferation of TcR V gamma 3 thymocytes. When combined with low concentrations of IL-2, a synergistic effect could be observed with IL-1, IL-4 or IL-7. It is shown that the synergistic effect of IL-2 with IL-4 was mainly due to induction of IL-2 receptor expression. The synergistic effect of IL-2 and IL-7 on the proliferation of TcR V gamma 3 cells could only be partially inhibited by anti-IL-2 receptor MoAb, and this antibody had no effect on the IL-2 + IL-1 cultures. These observations can explain the extensive proliferation of TcR V gamma 3 thymocytes during fetal life and they indicate that TcR V gamma 3 thymocytes have the potential to play a functional role during fetal thymus development.