Cultures of human lymphocytes obtained from blood of healthy adult donors were irradiated at different doses of gamma-rays (60Co), and irradiated cells were analysed at metaphase in 50 hours after the irradiation. The effect (total yield of aberrations of chromosome type, or total yield of exchange type aberrations) produced by the lowest dose (5 r) appears to be statistically significant in a sample of 1500 cells. Within usual dose-range (25--400 r) both parabolic and linear-quadratic equations give a satisfactory fit of experimental data (dicentrics, fragments, or all aberration of the chromosome type). Low doses of gamma-rays produce, however, more aberrations than expected, if one extrapolates dose-effect curves from higher doses. Both equations should be considered therefore just as empirical equations. Dicentrics show a plateau at low doses (10--30 r) which appear to be statistically significant. Some indications are obtained that the total number of chromosome-type aberrations is a more reliable criterion of cytogenetic damage than usually accepted yield of dicentrics and rings.