- An explant outgrowth technique to determine the radiation response of five different human organs (bladder, oesophagus, colon, breast and thyroid) is described. In each case except thyroid, where malignancies are rare, data are presented for normal and malignant tissue. Results show that variations in response, consistent with those observed in vivo, can be measured. Tumours were in all cases highly resistant to radiation relative to their corresponding normal tissue. Possible reasons for this are discussed. The method may prove useful in the prediction of the radiobiological response for tumour and surrounding normal tissue where post-operative therapy is planned.