Whether expression of transcription elongation factors is regulated during development has not been investigated, though genes encoding elongation factor S-II are transcribed in a tissue-specific manner. We investigated the expression profile of tissue-specific S-II during development using an isolated cDNA, termed mouse S-II-T1, whose transcripts are detected almost exclusively in testis. Three experiments were performed with various types of germ and somatic cells in testis to determine in which cells S-II-T1 is expressed. (1) Expression of S-II-T1 is markedly reduced in the testes of adult WBB6F1-W/Wv mutant mice which lack testicular germ cells, indicating its expression is specific to testicular germ cells. (2) The onset of mouse S-II-T1 mRNA appearance in testis is seen about 10-14 days after birth, which is consistent with the start of meiotic events, suggesting that S-II-T1 is not transcribed in premeiotic and early meiotic cells such as spermatogonia, leptotene spermatocytes, or zygotene spermatocytes. (3) Mouse S-II-T1 transcripts accumulate in meiotic pachytene spermatocytes and are detected in postmeiotic haploid cells such as round and elongated spermatids during spermatogenesis, as shown by fractionation of testicular germ cells at four different stages. These results indicate that expression of mouse S-II-T1 is restricted to testicular germ cells during and after meiosis in the course of spermatogenesis. This is the first report that expression of a transcription elongation factor in particular cells is regulated in a stage-specific manner in the course of development.