An attempt has been made to investigate the adaptive response to ionizing radiation in the human lymphocytes in vitro using cytochalasin-B blocked micronucleated binucleate cells (mn-BNCs) as a cytogenetic end point. Whole blood samples drawn from healthy donors, of either sex were irradiated in vitro at a dose of 1 cGy (adaptive or conditioning dose) Cobalt-60 gamma radiation (dose rate 1.12 cGy/min) at about 26 hr after mitogenic stimulation. After 31 hr of their initiation, groups of cultures were subsequently exposed to a challenging dose of 100 cGy gamma radiation (dose rate 82 cGy/min.). Eight males in the age group ranging from 25 to 55 years and eight females (age group 25 to 29 years), have been analysed during this study. Analysis of data revealed 40.6% reduction in the frequency of mn-BNCs among the males with a range from 25.7% to 54.7%. In case of females, also the per cent reduction varied from 26.3% to 49.0%, with a mean value of 33.7%. Pooling the data from males and females gave an overall reduction of 37.1% in the frequency of radiation induced mn-BNCs due to pre-exposure to 1 cGy radiation.