The injection of a syngeneic Gross-virus-induced lymphoma into W/Fu rats induced peaks of cytotoxicity in the spleen attributable to non-T cells and T cells 3 and 10 days later, respectively. The conditions required for augmenting the cytotoxicity of the non-T cells in various lymphoid compartments (shown elsewhere to closely resemble NK cells) were analysed using the ip and iv routes of inoculation and a variety of tumour cells including those normally susceptible or resistant to lysis by NK cells in vitro. Using an ip inoculation of W/FuG-1 cells (a tumour susceptible to lysis by NK cells), a short-lived, 3-fold increase in cytotoxicity was observed in the spleen at day 3 and a 5-fold increase in the PEC at day 5. Cytotoxicity in other lymphoid organs remained unchanged. Tumours resistant to lysis by NK cells also stimulated cytotoxicity in the spleen or PEC, although the effect depended on the dose and route of inoculation used, and depression of cytotoxicity was observed under some conditions.