Resembling the in vitro antibody response of the newborn cultures of cord blood lymphocytes stimulated with pokeweed mitogen (PWM) generated fewer plasma cells (PC) than comparable adult lymphocyte cultures and the response was almost exclusively of the IgM class. We investigated the cellular basis of this difference by preparing mixed cultures of newborn T or B lymphocytes with adult B or T cells. Substitution of adult for newborn T cells enhanced the response of newborn B cells, particularly of the IgG and IgA classes. The response of second trimester fetal spleen cells was also increased by adult T cells, although no IgA PC appeared. Conversely, adult B cells generated fewer PC particularly of the IgG and IgA classes when cultured with newborn T cells. The relatively poor IgA and IgG responses of newborn cells seems partially but not entirely due to deficiency of T cell helper function. Suppressor activity of newborn T cells was investigated by adding excess unrelated newborn or adult T cells to adult T + B cells: adult T cells improved the response whereas newborn T cells were variably suppressive. The results indicate that newborn T cells, although capable of helper function, are balanced toward suppression.