The present work shows that stimulation of peripheral blood lymphocytes with autologous or allogeneic B-cell lines leads to a strong induction of cytotoxic effector cells with spontaneous killer (SK) cell specificity, apart from the specific response directed against the particular stimulating cell. To demonstrate this we have determined a relative target cell specificity in the SK system, defined by the short-term 51Cr release assay, and established a relative specificity index (RSI). Using this approach we have been able to show that killer cells induced during a 5-day cocultivation period with B-cell lines have a similar PSI to that of unstimulated SK cells. In addition, we have shown that such killer cells can be induced from several different lymphocyte subpopulations and that they, in contrast to SK cells, do not express Fc receptors. The implications of these findings in relation to the nature, mechanism, and biological significance of the SK cell system is discussed.