The rat R-1 rhabdomyosarcoma with a capacity for colony growth in vitro after excision of the tumour and dissociation by a trypsin method has been used to investigate the effectiveness of radio-chemotherapy. Growth delay data were compared with data on survival of cells derived from tumours treated in situ. An excess in growth delay was observed when vinblastine (1.5 mg/kg) was given at intervals of 0.3 to 2 d after or 4 d before a dose of 20 Gy of X-rays. Cell survival data indicate that the maximum effectiveness of the drug treatment and the combined treatment (vinblastine and a dose of 10 Gy) can be assessed 2 to 3 d after treatment. The fractions of surviving cells determined after combined therapy at 0, 1 and 2 d intervals were not significantly different from the fractions expected on the basis of simple multiplication of the fractions surviving individual treatments. The data suggest that the excess in tumour growth delay observed cannot be accounted for by co-operative interaction of the doses of radiation and drug.