Spontaneous and transplanted tumour growth rates studied in the C3H mouse have shown that when only one spontaneous tumour was present in one strain then the distribution of growth rates closely resembled that for first generation isotransplants of another strain. It was also shown that the number of spontaneous tumours (in the range one to four tumours) present on the mouse affected the tumour growth rate, i.e. the more tumours per mouse, the slower the growth rate of the earliest tumour. This factor might partly account for the discrepancy between human tumour growth rates (normally determined when many tumours are present in the patients) and the faster tumour growth rates observed in experimental animals, in which normally only single tumours are present.