Human peripheral blood lymphocytes were obtained from healthy individuals and their cytotoxicity against retinoblastoma cells was measured using 51Cr-release assay. The lysis of two human retinoblastoma cell lines, Y-79 and WERI-Rb 1, varied greatly from blood donor to donor, but the lysis of K-562 was very similar among the donors. Furthermore, the lysis of Y-79 was markedly inhibited by the addition of K-562, a natural killer (NK) cell-sensitive target, but not by Raji, an NK-resistant target, suggesting that both Y-79 and K-562 shared a common NK-directed antigen. The major cytotoxic activity was associated with nonphagocytic and nonadherent cells, and it was finally recovered in the low-density fraction which was enriched with large granular lymphocytes. These lines of evidence indicate that natural killer cells (NK cells) in the peripheral blood killed retinoblastoma cell lines. On the other hand, three kinds of retinoblastoma cells freshly isolated from patients were resistant to the lysis by the lymphocytes obtained from the donors with high NK activity. However, these tumor cells displayed an enhanced susceptibility when the effectors from the same donors were treated with interferon. The study of NK cells and the interferon system may shed light on the treatment of retinoblastoma.