We show that by measuring the joint distribution of fluorescent and scattered light in a suspension of lung taken from the C57B1/6J mouse, we are able to discriminate in vivo between the lung metastases of the B16 melanoma and the normal lung tissue. We can in such a way detect metastatic cells in a very early stage of growth and also obtain growth curves for the metastatic population. We analyze the sensitivity of the method of detection and speculate how it might be used in aiding in human diagnosis.