ATTEMPTS to provide a physical separation of functional populations of lymphocytes have exploited a wide variety of cellular characteristics1–12. Work with other cell types has shown that different cells have different susceptibilities to damage during freezing and thawing13–18 which suggested that this stress could possibly be used as a selectively damaging procedure. Functional lymphocytes can be recovered after freezing19–23 and possible differences in the survival of populations of small lymphocytes frozen over a range of cooling rates in the presence of various concentrations of dimethylsulphoxide (DMSO) were investigated. Differential survival after thawing was assessed by stimulation with specific and non-specific stimulants using a microplate culture technique. Separation could only be achieved, however, by stimulating a sub-population of lymphocytes before freezing. Variation of the cooling conditions could be used either to damage or to protect the resulting activated cells in comparison with the unstimulated small lymphocytes.