AbstractThe primary in vitro response of spleen cells to 4‐hydroxy‐5‐iodo‐3‐nitrophenacetyl‐(NIP)‐ hapten was markedly reduced by procedures which depleted the spleen cell population of T cells. One approach was to use adult thymectomized, irradiated, bone marrow restored (TxBM) mice. The anti‐NIP response of spleen cells from carrier (SRBC)‐primed TxBM mice was significantly reduced compared to that of primed sham‐TxBM or primed normal spleen cells, and correlated with a deficient secondary anti‐SRBC response in vitro.Two criteria were used for T cell depletion of TxBM mice: their impaired ability to mount a primary response to SRBC in vivo and the lower proportion of their spleen and lymph node cells susceptible to the cytotoxic action of anti‐Θ serum.Using a second approach, the anti‐NIP response of normal carrier‐primed spleen cells was reduced by pretreatment of the cells with anti‐Θ serum and complement. The anti‐NIP response of anti‐Θ treated cells and cells from TxBM mice was enhanced by the addition of irradiated spleen cells from primed normal mice.The requirement for carrier‐priming to induce a good primary in vitro antihapten response was not abrogated by injecting donor mice with allogeneic lymphoid cells at various times before the preparation of cell cultures.