Till now the immunohistochemical studies on the procathepsin D expression in breast cancer have been performed using polyclonal antibodies on paraffin-embedded sections because the monoclonal ones were considered effective only on frozen sections. In this study we compared the MAb M1G8 reactivity, which recognizes the 52 Kd procathepsin D, on a series of frozen and paraffin-embedded sections of the same specimen from 22 breast cancers, using the immunohistochemical method. This methodological choice was due to the necessity to perform further studies with monoclonal antibodies using archival material to clarify the procathepsin D prognostic role. MAb M1G8 was clearly positive with a 100% concordance rate both in frozen and paraffin sections of the same specimen.