Mice irradiated with gamma-ray develop large numbers of Ig-containing cells of various isotypes in all the lymphoid tissues examined except the thymus. The maximal dose of irradiation is approximately 500 R/mouse. These indigenous Ig-containing cells start to appear at day 3 post-irradiation, peak at day 6, and subside thereafter. The expression of the indigenous Ig-containing cells can be enhanced in the spleen of recipient mice by passive transfer of various lymphoid cells, except thymocytes. Lymphoid cells from histocompatible donors are more effective enhancers than those from nonhistocompatible donors. Cells involved in the enhancement are primarily T cells. In contrast, when T cells are eliminated from the lymphoid cell inocula, the indigenous Ig-containing cells are reduced to a level lower than that in the irradiated controls. This suppression appears to be mediated by macrophages, since recovery was found when macrophages were removed from these T cell-depleted inocula.