Intracisternal A particles (IAP), murine endogenous retrovirus, make up 0.3% of the mouse genome. They are expressed in some normal tissues, certain transformed cell lines, and show stage-specific patterns of expression in early embryos. We have used peptide-specific antisera and the polymerase chain reaction to explore type-specific expression of these IAP during preimplantation development. In this paper we show that the IAP core protein, p73, characteristic of type IIAP, is present throughout preimplantation development while the gag-pol fusion protein p120, characteristic of the variant type I delta 1, is synthesized and expressed only from the 8-cell stage onward. Type IIAP RNA is present at all stages and appearance of p120 at the 8-cell stage could represent new transcription or translation from a preexisting I delta 1 message. The presence of type II IAP RNA varies according to stage, with two sizes of type II transcripts present at all stages except the 2-cell stage at which time only the smaller of the two transcripts can be detected. The reappearance of the larger type II transcript subsequent to the 2-cell stage implies new transcription of this type II subspecies. The presence of type I, II, and p73 in the unfertilized egg strongly suggests maternal inheritance from the oocyte.