Measurement of frequencies of 6-thioguanine-resistant (TGr) human peripheral lymphocytes may contribute to quantitative genetic risk assessment in occupationally or environmentally exposed human populations. A simple procedure for the detection of TGr human peripheral blood lymphocytes was developed in our laboratory, using whole blood culturing and 5-bromodeoxyuridine (BrdU) labeling in combination with immunocytochemical staining. Modifications of the procedure designed to reduce the false positive effects of spontaneously cycling lymphocytes (phenocopies), and to optimize duration of BrdU labeling and the culturing period, were evaluated. A standard procedure was developed which applied 24 h cold storage of the diluted heparinized blood (1:10, v:v in RPMI 1640 medium) at 4 degrees C to reduce the effect of spontaneously cycling lymphocytes, and whole blood culturing in RPMI 1640 complete medium with stimulation of T lymphocytes using phytohemagglutinin (PHA), selection of TGr lymphocytes by adding TG to a final concentration of 2 x 10(-4) M and, after 24 h of incubation, labeling of TGr lymphocytes with 2.5 x 10(-5) M BrdU during 16 h. Using this standard procedure, frequencies of TGr cells in 45 healthy individuals (aged 21-64) were observed to range from 0.3 to 229.8 x 10(-6), with a mean variant frequency (VF) (+/- SD) of 136 x 10(-6) (+/- 35.8). After exclusion of the one extremely high value of 229.8 x 10(-6), mean VF was 8.7 x 10(-6) (+/- 14.1). A significant inverse correlation was found between logVF and the labeling index of control cultures (LIc), indicating that cultures with low LIc tend to yield higher VF.(ABSTRACT TRUNCATED AT 250 WORDS)