The metastatic TS/A line has been recently derived from a spontaneous BALB/c mammary tumor. When TS/A cells were cultured in 0.33 per cent agar, two morphologically distinct types of colonies were observed from which two sets of clones were obtained. E clones were derived from small, transparent colonies, whereas F clones were from large, thick, actively growing colonies. All the clones were tumorigenic in syngeneic BALB/c females. However, E clones showed higher ability than F clones to metastasize spontaneously to the lung. Comparison between E and F clones shows that the high level of spontaneous metastasization to the lung is associated with epithelial-like in vitro growth pattern, spontaneous dome formation and growth pattern in 0.33 per cent agar cultures. The ability to give rise to lung colonies following intravenous inoculation is not a predictive parameter for the spontaneous metastatic potential.