22 different human ovarian tumours were heterotransplanted subcutaneously to female "nude" mice. When small tissue pieces were used 10 out of 16 tumours grew. Subcutaneous injections of suspended tumour cells were made to 11 mice: all failed. Metastatic or infiltrative growth was never seen in the mice observed for up to 25-40 days. Material from each tumour was transplanted to four separate sites in two mice. The successful grafts largely retained the original morphological features. As measured with H3-thymidine incorporation, DNA-synthesis was also similar in the original tumours and the growing grafts. This animal in vivo model might be highly informative for the study of inter alia fundamental biology, chemotherapy, and irradiation of ovarian tumours.