The relationship of lymphocyte proliferative capacity to age was studied using lymphocytes from neonates, from young adults aged 20-30 and from healthy individuals aged 70-90. Mass cultures expanded exponentially and eventually died after a final expansion of 10(19)-10(52). They therefore showed a Hayflick effect, but in contrast to reported findings for other cell types there was no relationship between age and the magnitude of the final expansion. Cytogenetic and molecular studies showed that monoclonality developed in all mass cultures. Study of individual clones also showed exponential growth followed by cessation. The magnitude of the expansion, 10(4)-10(35), was substantially less than that observed for mass cultures, but was related to age. We conclude that lymphocytes have a heterogeneous proliferative potential, that the overall proliferative potential declines with age but that rare cells having extended proliferative potential continue to be present into old age. The development of monoclonality during the course of mass cultures has implications for the interpretation of findings from such cultures since observations drawn from the later stages of culture reflect the properties of rare cells having high proliferative potential and do not necessarily reflect the properties of the overall population.