Cytochemical studies have been performed to investigate several enzymes which are involved in the metabolism of drugs, toxins, carcinogens and a variety of endogenous substances. Three groups of enzymes were studied: cytochrome P-450, gamma-glutamyl transpeptidase and NADP linked dehydrogenases. Also studied was glutathione, a peptide closely associated with these enzymes. The distribution of cytochrome P-450 in human tissues was determined by immunocytochemistry with monoclonal antibodies to a major constitutive form of human hepatic cytochrome P-450. Within the liver, cytochrome P-450 was present predominantly in hepatocytes of zone 3 of the liver acinus. Cytochrome P-450 was also demonstrated in the columnar absorptive cells of small intestinal villi, polymorphonuclear leucocytes and their precursors and mast cells. A novel cytochemical technique for demonstrating glutathione was developed. Glutathione was identified using o-phthaldialdehyde to produce a fluorophore which was visualised with fluorescence microscopy. This method was used to study the localisation and distribution of glutathione in breast lesions, particularly invasive carcinomas, where a variable level of glutathione in different carcinomas was demonstrated. A method was devised for the enzyme histochemical demonstration of gamma-glutamyl transpeptidase in fixed resin embedded tissue. Enzyme activity was optimally maintained and accurately localised in tissue displaying excellent morphology by fixing tissue in cold methanol free formaldehyde and then embeddding the tissue at low temperature in glycol methacrylate resin. Several NADP linked dehydrogenases and NADPH-cytochrome P-450 reductase were localised by enzyme histochemistry in sections of freeze-dried resin embedded tissue. This novel method of tissue preparation was evolved to allow the demonstration of the activity of dehydrogenases in resin embedded tissue.