The distribution of glutathione S-transferase T1 (GSTT1) phenotypes was studied in a total sample of 673 Estonians whose four grandparents were born in Estonia, by an ELISA test able to differentiate between GSTT1 positive and GSTT1 negative phenotypes. 18% of the total sample did not present GSTT1-1 protein in whole blood. GSTT1-1 concentration was assayed in 519 out of the 552 GSTT1 positive subjects (i.e. 82% of the total sample) 49% percent of this subsample made up by 519 subjects was found to have GSTT1-1 in intermediate concentration and 33% in high concentration. The gene frequency of the GSTT1 deleted allele was estimated to be 0.423 as the square root of the frequency of the GSTT1 negative subjects (square root of 0.18 = 0.423) and that of the GSTT1 positive allele as (1-0.423) = 0.577. Statistically significant regional differences were found within the population with the lowest frequency of GSTT1 negative in western Estonia (9.5%) and the highest in the southeastern part of the country (24.5%).