Thymocytes were removed from mice at different times before and after birth, and their phenotype and killing repertoire were analysed after stimulation with human recombinant interleukin 2 (IL-2). Whereas three different tumour targets were killed by all lymphokine-activated killer (LAK) populations tested, EL4 lymphoma cells were only killed by LAK cells derived from thymocytes after, but not before, birth. Cold-target competition tests showed that LAK thymocytes recognized antigenic structures on EL4 cells that are different from those on other tumour targets. Analysis of EL4 target killing after removal of different subsets from LAK cells revealed that the major part of this killing is exerted by CD8+ cells. Our findings are additional and more direct evidence for partial target specificity in LAK cells.