We previously described morphological, functional and antigenic differences between a chemically-induced DBA/2 lymphoma, Eb, and a spontaneous variant, ESb, which arose in 1968 and had highly increased metastatic capacity. Now we present evidence that the two cell lines, in spite of the differences observed, are still related. (1) Shifts from Eb to ESb can be reproduced after 11 years. They occur during routine i.p. transplantation, especially when high cell numbers are passaged. (2) ESb variant cells can be recovered from Eb tumor populations after immunoselection in vivo with specific anti-Eb cytolytic T lymphocytes (CTL). (3) Parental-type Eb cells can be isolated from ESb tumor populations after immunoselection in vitro with specific anti-ESb CTL Ten ESb cells mixed with 10(6) Eb cells and inoculated s.c. caused a shift in the mortality curve suggesting that a possible contamination of the Eb population with pre-existing ESb variant cells was less than 1 in 100,000.