Previous data reported that connexin 43 (Cx43) is the most predominant gap junction protein found in the testis and is essential for spermatogenesis. In the present study, In situ hybridization and indirect immunofluorescence were developped to analyze Cx43 mRNA and protein in germ cells of men with normal spermatogenesis and in human testicular seminoma. In testis with normal spermatogenesis Cx43 mRNA were basally located within seminiferous tubules. Semiquantitative analysis showed that Cx43 mRNA were similarly expressed in the most basally located and less mature germ cells (spermatogonia, early spermatocytes, and pachytene spermatocytes) and in the supporting Sertoli cells. No hybridization signal was found in round and elongated spermatids. Immunofluorescence analysis showed that Cx43 signal was mainly located in the basal compartment of seminiferous tubules. As observed for normal germ cells Cx43 transcripts were detected in human testicular seminoma sections. Silver grains were only present within the cytoplasm of seminoma cells identified by PLAP staining and were absent in contaminant cells. Immunofluorescence studies indicated that Cx43 staining was distributed in the intracellular compartment rather than at the plasma membrane level as expected. Taken together these data represent the first demonstration of cellular localization of Cx43 mRNA in human specific germ cells and suggest traductional rather than transcriptional abnormalities in human testicular seminoma.