The biological properties and metastasis of two sublines of the Lewis lung carcinoma (3LL) which have maintained a stable pattern of organ-selective metastasis have been studied. Subline M-3LL, a lung-specific variant which originated from a lung metastasis of the parent line, metastasized only to the lung following injection of 10⁴–10⁶ tumor cells i.v. or s.c. Lymphatic metastases of this tumor were rarely detected. Subline H-3LL which was developed from a rare, spontaneous hepatic metastasis of the parent line metastasized primarily to the liver, but pulmonary metastases have also been observed. While it grew at local s.c. sites, this tumor metastasized to the regional lymph nodes draining the tumor site, as determined by histology and by bioassay of the lymph nodes following their grafting into new recipient animals. Histologically, the two lines were indistinguishable with the exception of a higher incidence of giant cells detected in tissue sections and culture monolayers of the liver-colonizing variant H-3LL. Ten clones derived from each of the variant lines were expanded in vitro and inoculated i.v. While none of the ten clones derived from line M-3LL gave rise to extrapulmonary metastasis, nine of ten clones derived from Tumor H-3LL gave rise to hepatic metastasis. Highly metastatic clones selected from each tumor were subsequently used to study the patterns of distribution and arrest of radiolabeled tumor cells following their inoculation i.v. No correlation could be found between the initial distribution of the radiolabeled tumor cells and the organ selectivity eventually noted in the site of the metastases.