Human hematopoietic target-cell lines known to be lysable in short-term 51Cr-release assays by natural killer (NK) cells occurring in spleens of normal, nonimmune CBA/J and A/J mice were adapted to grow in culture medium supplemented with human AB serum or with normal mouse serum (NMS). After a culture period of more than 8 weeks, the resulting AB serum- or NMS-grown sublines exhibited the same degree of sensitivity to mouse NK cells in direct or competition assays as the original lines kept in fetal calf serum. Susceptibility of the various target cell lines was also independent of the type of serum supplement used during the cytotoxicity assay. Preabsorption of the serum supplement with in vitro cultured cell lines identical to the target cells left NK cytotoxicity to these lines intact. In this heterologous system, no changes in the expression of NK relevant target structure(s) could be detected in sublines grown in sera from different species, but target susceptibility seemed to be preserved.