Glutathione S-transferase (GST) class Mu activity was determined in 145 unrelated hospital patients in Berlin by measuring their conjugation activity towards the specific substrate trans-stilbene oxide (TSO) with two substrate concentrations (50 and 250 microM) in homogenates prepared from lymphocytes. Eighty individuals (55.2%) had an activity lower than 10 pmol/min/10(6) lymphocytes and were classified as GST class Mu deficient. In 142 of 145 cases, phenotype was confirmed by the results of a genotyping procedure using the polymerase chain reaction technique. Two fragments of 273 and about 650 bp including one and two introns, respectively, could always be amplified from genomic DNA in individuals with high GST class Mu activity and could not be amplified in persons with impaired glutathione-TSO conjugation activity. This indicates that persons with low activity carry a large deletion mutation within the GST class Mu gene. The enzymatically determined antimode between low and high activity determined as 10 pmol/min/1 million lymphocytes in the assay with 50 microM TSO could be clearly confirmed by genotyping.