The seminiferous tubular fluid (STF) provides the microenvironment necessary for spermatogenesis in the adluminal compartment of the seminiferous tubule (ST), primarily through secretions of the Sertoli cell. Earlier studies from this laboratory demonstrated the presence of glutathione S-transferase (GST) in STF collected from adult rat testis and in the spent media of ST cultures. This study describes the cellular source, isoform composition and possible function of GSTs in the STF. The major GST isoforms present in STF in vivo share extensive N-terminal similarity with rat GSTM1 (rGSTM1), rGSTM2, rGSTM3 and rGST-Alpha. Molecular masses of rGSTM2, rGSTM3 and rGST-Alpha from liver and testis sources were similar, unlike STF-GSTM1, which was larger by 325 Da than its liver counterpart. Peptide digest analysis profiles on reverse-phase HPLC between liver and STF isoforms were identical, and N-terminal sequences of selected peptides obtained by digestion of the various isoforms were closely similar. The above results confirmed close structural similarity between liver and STF-GST isoforms. Active synthesis and secretion of GSTs by the STs were evident from recovery of radiolabelled GST from the spent media of ST cultures. Analysis of secreted GST isoforms showed that GST-Alpha was not secreted by the STs in vitro, whereas there was an induction of GST-Pi secretion. Detection of immunostainable GST-Mu in Sertoli cells in vitro and during different stages of the seminiferous epithelium in vivo, coupled with the recovery of radiolabelled GST from Sertoli cell-culture media, provided evidence for Sertoli cells as secretors of GST. In addition, STF of 'Sertoli cell only' animals showed no change in the profile of GST isoform secretion, thereby confirming Sertoli cells as prime GST secretors. Non-recovery of [35S]methionine-labelled GSTs from germ cell culture supernatants, but their presence in germ cell lysates, confirm the ability of the germ cells to synthesize, but not to release, GSTs. Functionally, STF-GSTM1 appeared to serve as a steroid-binding protein by its ability to bind to testosterone and oestradiol, two important hormones in the ST that are essential for spermatogenesis, with binding constants of <9.8x10(-7) M for testosterone and 9x10(-6) M for oestradiol respectively.