A total of 241 serum samples from 145 parous tammar wallabies (Macropus eugenii) were screened for presence of antibodies to paternally derived antigens of the fetus. These samples were taken at different stages in late pregnancy after placental contact was intimate and after birth. Complement-dependent cytotoxicity tests were unable to detect any specific antibodies. It is concluded that the yolk sac placenta of M. eugenii does not allow intimate enough contact between fetal tissues and the maternal circulation to induce formation of cytotoxic antibodies by its mother. This is in contrast to eutherian mammals, in which such production of cytotoxic antibodies occurs frequently as a result of pregnancy. Together with other data it is suggested that the short implantation period in M. eugenii, which is common to all marsupials, has probably not evolved to prevent maternal immune attack upon the conceptus.