EGF receptors were identified on peritubular cells and Leydig cells of normal human testis using immunohistochemistry with monoclonal antibodies EGF-R1, EGF-R and F4. In a radioligand exchange assay the binding site for EGF on human testicular tissue was characterised as having a Kd of 1.18 ± 0.32 nM with approximately 528±116 fmoles binding sites per mg of protein. The molecular weight of the receptor/ligand complex, identified by crosslinking and SDS-page gel electrophoresis was approximately 125 kDa. This may represent a proteolysed form of the receptor complex. Competition studies illustrated that of the peptides tested, only EGF and TGF-α were specific for the EGF binding site. In cancerous testicular tissue EGF receptors were not identified on seminoma and Leydig cell tumours but they were located, by both immunohistochemical and biochemical techniques on teratoma tumours. The EGF receptor on teratoma cells was characterised using the Tera-2 cell line. The EGF receptor was a high affinity site with a Kd of 0.21±0.08 nM, with approximately 6.73±0.81 x 10^4 binding sites per cell. Molecular characterisation was performed by Western Blot Analysis employing the monoclonal antibodies EGF-R1 and F4. The receptor was identified in both the 170 and 125 kDa form. Competition studies with other peptides clarified once again the specificity which EGF and TGF-α have for the receptor site. EGF and TGF-α were both present in normal testicular tissue at 5.16±0.97 ng and 2.76±0.15 ng per gram of wet human testicular tissue respectively. The concentrations of androstenedione, DHT and testosterone in human testicular tissue were 0.46±0.13 nmoles, 0.29±0.06 nmoles and 15.58±2.55 nmoles per gram of dry human testicular tissue, respectively. EGF and TGF-α were also secreted by Tera-2 cells at concentrations of 2.40±0.33 pg and TGF-α at 2.55±0.78 pg per ml of culture media. TFG-α and EGF both competed for the EGF binding site previously identified on the Tera-2 cells. EGF did not alter thymidine incorporation by the cells when incubated over 24 hours at concentrations ranging from 0.3 to 100 nM. TGF-α however, increased thymidine incorporation by approximately 2-fold when employed at concentrations greater than 3 nM over 7 hours in culture. The maximum increase in thymidine incorporation was apparent after 24 hours employing a concentration of 100 nM TFG-α. Mybolerone was found to have no effect on thymidine incorporation or EGF receptor expression when Tera-2 cells were incubated in concentrations from 0.3 to 100 nM for 24 hours. It was therefore postulated that Tera-2 cells are not androgen sensitive. The results of these studies indicate that EGF and TGF-α may be involved in testicular cell-cell communication and in the development of testicular cancer.