GnRH biological effect is mediated through specific GnRH membrane receptors (GnRH-receptor, GnRH-R) that have been demonstrated in human breast cancer by molecular and biochemical techniques. The A9E4 monoclonal antibody (moAb) against the epitope 1-29 of N-terminal of human GnRH-R has been proposed, suggesting the possibility to perform retrospective studies for the confirmation of clinical relevance of this receptor. The aim of the present study was to verify the performance of the A9E4 moAb when utilised for immunohistochemical analysis in 71 formalin-fixed paraffin-embedded breast cancer samples; furthermore, a comparison with results obtained with the radioligand biochemical assay (GnRH-Rbca) and with Western blot has been performed. The A9E4 specificity was preliminarily demonstrated by Western blot analysis in both MCF-7 and T47D breast cancer cell lines. In both cell lines, only a protein of 60-64 kDa was demonstrated in the membrane and nuclear compartments. Immuno-reactivity for A9E4 was detected in the cytoplasm of morphologically normal adjacent glandular epithelia and in tumour cells. Cytoplasmic GnRH-R immuno-staining (GnRH-Rica) was shown in 55% of tumours but only 28% of these had a percentage of positive cells higher than >25%. A correlation between the percentage of positive GnRH-Rica cells and femtomoles of the GnRH-Rbca content was shown (c.c.=0.295, p=0.01). The mean content of GnRH-Rbca in the subgroup of tumours with >25% of cell positive at GnRH-Rica was significantly different with respect to that of negative GnRH-Rica tumours (25 fmol vs 11 fmol, respectively; p=0.03 by t-test). The immunohistochemical analysis of GnRH-R by A9E4 moAb in human breast cancer tissues seems to provide information that correlates with the standard biochemical assay. Retrospective clinical studies with GnRH-Rica on archival samples are strongly suggested to verify the prognostic-predictive relevance of this receptor in human breast cancer.