The ability of mammalian species to respond to in vivo challenges with complex antigens develops in a time dependent manner during embryonic and neonatal life. Functional or structural immaturity of B cells, T cells, and accessory cells in fetal or neonatal animals as well as active, non-specific suppression of humoral and cellular immune responses by neonatal immunocytes has been reported. Utilizing two systems, 1) the in vitro primary antibody response to soluble hapten-protein conjugates, and 2) the one way mixed lymphocyte response, it has been shown that thymic T cells and both splenic T and non-T cells are capable of actively suppressing immune response by cells from adult animals. The splenic adherent cell population (SAC) isolated from neonates was also shown to be an inefficient antigen presenting accessory cell. These neonatal SAC were also inefficient stimulators of MLR across both H-2 and Mls differences. Fluorescent microscopic studies revealed that fewer neonatal SAC were Ia positive than adult SAC. Moreover the development of functional accessory cell activity with age paralleled the expression of Ia antigens by SAC.