Twenty-three human haematopoietic cell lines, normal and mitogen stimulated peripheral blood lymphocytes and tumour material from fresh leukaemias, myelomas and lymphomas were investigated with a panel of cytochemical reactions. Normal and mitogen stimulated lymphocytes, non-neoplastic lymphoblastoid cell lines (LCL), lymphoma lines with B-lymphocyte characteristics, chronic lymphocytic leukaemia and fresh lymphocytic lymphomas reacted weakly or negatively with all stains. T-lymphocyte acute leukaemia lines were PAS and alpha-naphtyl acetate esterase positive. Myeloma lines and fresh myelomas were strongly beta-glucoronidase positive. A histiocytic lymphoma cell line was strongly esterase positive with naphtol AS-D acetate esterase inhibited by NaF. The three fresh histiocytic lymphomas, however, reacted as the lymphocytic lymphomas suggesting a lymphoid origin. A myeloid leukaemia line was strongly positive for acid phsophatase. No major disagreement was noted between the reactivity of established neoplastic lines and the corresponding fresh biopsy cells indicating an unaltered qualitative expression of enzyme production after prolonged in vitro culture.