During the preimplantation stages of pregnancy, the mammalian embryo is surrounded by an acellular envelope, the zona pellucida. Although little is known of the functions of this membrane it has been suggested that it may be impermeable to antibody molecules (immunoglobulins), thereby protecting the embryo from the potentially adverse effects of maternal immunity (Simmons & Russell, 1966, 1967). In the present study, we have investigated the permeability ofthe mouse zona pellucida to immunoglobulins by comparing the complement- mediated cytotoxic effect of rabbit anti-mouse serum and two of its immuno- globulin components, IgG and IgM, on the development of zona-free and zona-enclosed embryos in vitro. Mouse embryos at the 8-cell stage were recovered in phosphate-buffered saline (PBS) from the oviducts of randomly bred hybrid mice 48 hr after the finding of a vaginal plug. Half of the embryos were left with the zona pellucida intact, and the other half were transferred to 0\m=.\5 % pronase in PBS and in- cubated at 37\s=deg\C until partial digestion of the zona occurred, when removal was completed by mechanical disruption with a glass micropipette in pronase-free PBS. After a short recovery period in culture medium (RPMI + 10% fetal calf serum), groups of embryos, zona-free and zona-enclosed, were preincuba- ted for 1 hr at 37\s=deg\C in various dilutions of either whole rabbit anti-mouse serum or its IgG- and IgM-containing fractions obtained by ammonium sulphate precipitation followed by ion-exchange chromatography on DEAE cellulose DE 23. Following exposure to serum, embryos were washed twice in PBS and transferred either to culture medium containing 5% detoxified guinea-pig serum as a source of complement (Cohen & Schlesinger, 1970) or to culture medium alone. Control cultures in medium+ complement but without preincubation in antiserum were also carried out. To determine the time taken for antibody to cross the zona pellucida, a second series of experiments was performed in a similar manner except that preincubation in antiserum was varied from 1 to 30 min. All cultures were carried out in 0-1-ml volumes in Cooke Microtitre plates under 5% C02 in air, and were observed for 48 to 72 hr for evidence of cyto¬ toxic effect or development to morula and blastocyst stages. The results are summarized in Tables 1 and 2. Over the effective range of antiserum dilutions (1/10 to 1/1500), 8-cell mouse embryos treated with anti¬ serum and complement underwent destruction, without further development,