Abstract. T3 supplements enhance splenic primary thymus-independent antibody responses in the mouse in vivo. The purpose of the present investigation was to determine whether this effect may be mediated, in part, by direct influences on the lymphocytes and/or accessory cells involved in the response. A range of T3 levels (3 × 10−10 to 10−5 mol/l) was tested in microcultures of separated spleen cells from CBA/J mice 33 days of age. The immune reaction examined in vitro was the primary antibody response to trinitrophenylated Brucella abortus (TNP-BA). T3 was without influence, throughout the concentration range tested, on the number of anti-TNP plasma cells generated per culture. This result was obtained using splenocytes either from well-nourished or from malnourished mice, and using both optimal and suboptimal numbers of TNP-BA. On the basis of the present results and a reinterpretation of previous published work, it is concluded that the influence of T3 supplements on splenic antibody responses in vivo is mediated indirectly. Direct influences of T3 on the T-independent antibody response, if such occur, must be maximized by subphysiological levels of the hormone.