Inhibin B is a testicular peptide hormone that regulates FSH secretion in a negative feedback loop. Inhibin B is a dimer of an α and a β_B subunit. In adult testes, the cellular site of production is still controversial, and it was hypothesized that germ cells contribute to inhibin B production. To determine which cell types in the testes may produce inhibin B, the immunohistochemical localization of the two subunits of inhibin B were examined in adult testicular biopsies with normal spermatogenesis, spermatogenic arrest, or Sertoli cell only (SCO) tubules. Moreover, using in situ hybridization with mRNA probes, the mRNA expression patterns of inhibin α and inhibin/activin β_B subunits have been investigated. In all testes, Sertoli cells and Leydig cells showed positive immunostaining for inhibin α subunit and expressed inhibin α subunit mRNA. Using inhibin β_B subunit immunoserum on testes with normal spermatogenesis and with spermatogenic arrest, intense labeling was located in germ cells from pachytene spermatocytes to round spermatids but not in Sertoli cells. Inhibin β_B subunit mRNA expression was intense in germ cells from spermatogonia to round spermatids and in Sertoli cells in these testes. In testes with SCO, high inhibin β_B subunit mRNA labeling density was observed in both Sertoli cells and Leydig cells, whereas β_B subunit immunostaining was negative for Sertoli cells and faintly positive for Leydig cells. These results agree with the recent opinion that inhibin B in adult men is possibly a joint product of Sertoli cells and germ cells.