EC1, a clone from a cDNA library of embryonal-carcinoma (EC) cells, was used to study the expression of the gene(s) homologous to this sequence in murine germ cells and somatic tissues. Northern-blot analysis was used to determine the size and relative abundance of ECl transcripts in EC cells, ovary, testis, liver, spleen, brain, thymus, and bone marrow. The stage-specific expression of EC1-related transcripts in meiotic cells was examined in enriched populations of testicular cells (meiotic prophase spermatocytes, early and late spermatids, and residual bodies). All tissues and cells examined contained mRNAs of approximately 1.3 and approximately 3.6 kb which were homologous to EC1. In RNA from liver and thymus, a faint, lower-molecular-weight transcript of 0.75 kb was seen. A unique pattern of expression was observed in male germ cells. In cells that had completed meiosis, two additional transcripts of 1.0 and 4.4 kb were visualized, as well as high levels of expression of the 1.3- and 3.6-kb species. Only the 1.3- and 3.6-kb transcripts were detected in ovarian RNAs. The high level and stage specificity of expression of EC1-related mRNAs in germ cells suggest that this gene may be important in meiosis or in germ-cell maturation.