The aim of this study was to investigate the effect of microwave oven heating for antigen retrieval on the immunoreactivity of human prostate carcinoma androgen receptor (AR) in tissue sections. Formalin-fixed, paraffin-embedded tissue sections were microwaved at 5-min intervals for a total of 15, 20, 30, 35 minutes at maximum power (700W). The monoclonal antibody F39.4.1 directed against human AR was used at a 1:10 dilution. Without microwave oven heating prostatic tissue did not exhibit any AR immunoreactivity. Moderate positivity appeared after three 5-minute cycles of microwave heating. The intensity of immunoreactivity improved progressively with heating times of 20 and 25 min up to an optimum time of 30 minutes, when nuclear staining was most intense with the absence of background staining and without loss of morphological details. While antigen retrieval is effective in restoring antigenicity in a variety of setting, the length of time prostate tissue is exposed to microwave radiation is critical in order to obtain optimal AR immunostaining. AR immunostaining reliably permitted evaluation of the distribution and intensity of positively stained nuclei and the distinction of the various cell types in archival material.