In many species, the early post-fertilization development of the egg appears to occur mainly under maternal control and does not require transcription of the embryonic genome. In the mouse this situation is restricted to the one-cell stage; activation of the embryonic genome occurs at the late two-cell stage and results in a drastic change in the spectrum of proteins synthesized. This activation is preceded by a decrease in the overall synthesis of proteins at the end of the one-cell stage and the appearance, at the early two-cell stage, of a set of new polypeptides of molecular weight approximately 70,000 (70K) (refs 2, 8, 9). This can be compared with the series of events that occur after hyperthermia in differentiated cells. Heat shock results in an arrest of most transcription and translation; subsequently, expression of a limited set of genes, the heat shock genes, precedes the overall reactivation of cellular genome. Here we show that the 70K early two-cell-specific proteins are identical to two of the mouse heat shock proteins, HSP 68 and HSP 70.