Spheroids of V79 cells were subjected to fractionated irradiation with two doses of gamma-radiation. In addition, a two hours hyperthermic treatment at 42 degrees C immediately following the first dose was applied. Cycling and resting cells of this in-vitro tumour model were then assayed for survival as function of the fractionation interval. In parallel, changes in cycle progression between the doses were measured by means of cytofluorometry. As main proliferative effects induced by this combined radiation and heat treatment transient S-phase blocking of cycling and recruitment of resting cells were observed. The split-dose survival curve displayed considerable synergistic action of heat and radiation and a six hours delay of Elkind-recovery in both cycling and resting cells.