The Semipalatinsk area is highly contaminated with radioactive fallout from 40 years of continuous nuclear testing. The biological effects on human health in this area have not been studied. Significant remaining radioactivities include long-lived radioisotopes of 238,239,400Pu, 137Cs and 90Sr. To evaluate the long-term biological effects of the radioactive fallout, the incidence of micronuclei in lymphocytes from residents of the area was observed. Blood was obtained from 10 residents (5 females and 5 males, aged 47 to 55 years old) from each of the 3 areas of Znamenka, Dolon and Semipalatinsk, which are about 50-150 km from the nuclear explosion test site. For micronucleus assay, PHA-stimulated lymphocytes were cultured for 72 h and cytochalasin B was added at 44 h for detecting binuclear lymphocytes. Five thousand binuclear lymphocytes in each resident were scored. The means of micronucleus counts in 1,000 lymphocytes in residents of Semipalatinsk, Dolon and Znamenka were 16.3, 12.6, and 7.80, respectively, which were higher than those of the normal Japanese persons (4.66). These values were equivalent to the results obtained from 0.187-0.47 Gy of chronic exposure to gamma-rays at a dose rate of 0.02 cGy/min. The high incidence of micronuclei in residents of the Semipalatinsk nuclear test site area was mainly caused by internal exposure rather than external exposure received for the past 40 years.