The response of lung metastases of the mouse Lewis lung tumor to a single dose of cyclophosphamide (300 mg/kg) was studied as a function of the size of the metastases. Artificially induced and naturally arising (true) metastases were studied in the absence of a primary tumor. Artificial metastases exhibited a change in cure probability from 90 to 37% over a size range of a factor of 10. The true metastases had a larger spread in size for a similar change in cure probability but there were large uncertainties in the data and the results are not inconsistent with those for artificial metastases. Studies of cell kinetics of metastases at various sizes in the critical range for curability failed to demonstrate any marked differences. The results of drug treatment could be explained on the basis of an increase in cell number in the larger metastases.