Several monoclonal antibodies against human liver glutathione S-transferase mu were developed. One of these monoclonal antibodies, called GST-3H4 was further characterized and used in this study. In hepatic tissue, after immunoblotting, GST-3H4 strains a 27 kDa protein with a pI value of 6.2. GST-3H4 recognizes other human class-mu glutathione S-transferases, but does not detect acidic or basic glutathione S-transferases. By immunodetection with this monoclonal antibody, glutathione S-transferase mu can be demonstrated in human breast, stomach, liver, small and large intestine, mononuclear blood cells, kidney and placenta. A 100% correlation is found in the distribution of glutathione S-transferase mu when different tissues or mononuclear blood cells from the same individuals are investigated. In 62.5% of the mononuclear blood cells from controls, glutathione S-transferase mu is present. In patients with polyposis coli, breast cancer or colon cancer a similar distribution is found. Therefore no important role for glutathione S-transferase mu deficiencies in the aetiology of these diseases is suggested.