The induction of unstable chromosome aberrations in human peripheral blood lymphocytes exposed in vitro to protracted doses of cobalt-60 radiation is presented. Four dose response curves have been produced using constant exposure times of 1, 3, 6, and 12 h. The data fit well to the linear quadratic model and the yield coefficients have been compared with those obtained for acute (< 10 min) exposure. The quadratic coefficient is dependent on irradiation time and decreases approximately as predicted by Lea and Catcheside's G-function hypothesis. The possibility of a small proportion of much longer lived breaks is discussed. For purposes of biological dosimetry it is sufficient to assume a mean time of 2 h and a single exponential function for the repair of lesions when relating the effects of brief and protracted exposure.