Printed in Great Britain © Company of Biologists Limited 1978 The presence of F9 antigen on the surface of mouse embryonic cells until day 8 of embryogenesis

The fate of F9, a surface antigen common to embryonal carcinoma cells and cleavage embryos, as well as to male germ line cells has been studied in post-implantation mouse embryos. The antigen is readily detected on the surface of 7-day and 8-day, but not on 9-day embryonic cells, using anti-F9 serum absorption experiments and indirect immunofluor-escence staining. In addition, it is shown that an anti-F9 serum absorbed with 8-day (but not with 9-day) embryonic cells does not react with cleavage embryos (morulae). It is con-cluded that the antigenic determinants, which are recognized on the surface of morulae by the anti-F9 serum, persist until day 8 of embryogenesis, but are not detected on day 9, either due to their absence, masking or quantitative reduction