Fetal lambs were grafted in utero with orthotopically placed split-thickness skin homografts. At the same time the lambs, being essentially agammaglobulinemic, were given large intravascular doses of rabbit anti-sheep γ- and β2M-globulins, the levels of which were maintained in the fetal circulation by an appropriate intracardiac booster injection halfway through the experiment. The presence of persisting levels of these rabbit antibodies in the fetal circulation had no detectable inhibitory effect on the course of homograft rejection by the fetus. It is concluded from these data that conventional circulating antibody does not play an obligatory role in the rejection of solid tissue homografts. It is further suggested that the antibody or antibody-like activity responsible for the specificity of immunologically active lymphoid cells resides not on the surface but rather within the cell.