The induction of an adaptive response was obtained using mitomycin C (MMC) as both stimulating and challenging agent. Human lymphocyte cultures of two female donors were treated with 5, 10 and 20 ng/ml of MMC as conditioning doses. For the challenging treatments two different protocols were used (200 ng/ml for 4 h, and 400 ng/ml for 1 h). The scoring of sister chromatid exchanges (SCE) in the first challenging combination showed the following inhibition related with the expected SCE damage: 49.2%, 51.4%, and 36.9% for one donor, and 42.0%, 38.6%, and 34.7% for the other (corresponding to the stimulating dosages 5, 10, and 20 ng/ml, respectively). The second challenging combination gave an inhibition of 53.8%, 40.5% and 30.2% in one donor and 43.2%, 45.9% and 30.3% in the other donor.