The pattern of protein synthesis in 8-, 9- and 10-day post coitum (p.c.) mouse embryos was examined by 2-D gel electrophoresis of [35S]methionine-labelled proteins. Of the 600–800 polypeptides detected only one, a 14 X 10(3) Mr (14K) protein, was found to accumulate over this period. To isolate cDNA clones that potentially encode this protein, 32P-labelled cDNA was synthesized from 9 and 10 days p.c. embryo poly(A) +RNA, and used for the differential screening of an 8.5-day p.c. mouse embryo cDNA library cloned in lambda gt10. Six clones that hybridized strongly to the 10-day probe were purified and their inserts subcloned into plasmid vectors. Cross hybridization and restriction mapping of these inserts indicate that they fall into four distinct groups. Each of these hybridize with transcripts of approximately 600 nucleotides, which accumulate in the embryo from 9 to 10 days p.c. Expression was barely detectable in adult tissues and restricted to liver and spleen. Expression of one of these clones, 10.1, was examined by in situ hybridization of 35S-labelled RNA probes to 8.5-12.5 day p.c. embryo sections. Strong hybridization was observed in yolk sac blood islands, fetal liver and embryonic erythrocytes, suggesting that 10.1A encodes an erythrocyte-specific protein. DNA sequence analysis indicates that the four classes of cDNA were derived from transcripts of the alpha 1, zeta, beta h1 and epsilon globin genes. Labelling of 10-day p.c. erythrocyte proteins with [35S]methionine, followed by 2-D gel electrophoresis, clearly demonstrates that the most abundant polypeptide migrates to the same position as the 14K protein which accumulates from 8 to 10 days p.c. Thus the only abundant transcripts and corresponding proteins that change over a period of profound morphogenetic change correspond to globins of the newly established blood system.