A large granular lymphocyte (LGL) leukaemia cell line from the Fisher/F344 rat strain called RNK-16 has been established in vitro, maintaining the same surface markers as tumour cell growing in vivo. The tumour has also maintained its specificity pattern and cytotoxic reactivity and serves as a suitable source of natural killer (NK)-like effector cells in vitro. The cells show no evidence of dependency on, or production of, interleukin 2 or interferons, nor is the cytotoxic capacity influenced by treatment with mitogens. The in vitro line does not produce natural killer cytotoxic factor (NKCF) in a constitutive manner, but can be induced to do so via coculture with tumour target cells. When the fine specificity patterns were analysed, the RNK-16 cells express species-preferential lysis of susceptible target cells and a highly discriminatory power to kill only 1 out of 5 rat erythroleukaemia cell lines. When testing normal target susceptibility patterns, RNK-16 kills lymphoblasts of B type better than T blasts, which is well in line with previous findings on normal NK cell specificity patterns.