A case of T cell prolymphocytic leukaemia of helper/inducer T cell phenotype with IgM hypergammaglobulinaemia in a 65‐year‐old Saudi man is presented. The neoplastic T cells in the patient had an OKT3+, OKT4+, OKT8−, OKT11+, OKCLL+, OKIa1+, OKDR+, Tac+phenotype. The presence of OKIa1+, OKDR+, and Tac+antigens indicates that the leukaemic T cells were in an activated state. The cells responded to phytohaemagglutinin, but showed a diminished response to concanavalin A. The reduced interleukin 2 receptor expression and interleukin 2 production were associated with defective concanavalin A‐induced proliferation. There was suppression of mixed lymphocyte culture reaction between the patient and two healthy donors in response to concanavalin A. In vitro immunoglobulin production experiments demonstrate that the leukaemic T cells provided an enhanced helper cell function to produce IgM, and suppressor cell function to produce IgG immunoglobulins by pokeweed mitogen‐induced normal B lymphocyte differentiation. In this study we also found that the patient's T cells proliferate in response to lipopolysaccharides, thus providing the first evidence that leukaemic (OKT4+) T cells from a prolymphocytic leukaemia patient can be stimulated by lipopolysa