Over 200 established human hematopoietic cell lines of normal and malignant origin have been investigated by morphological and functional parameters. Employing morphology as the overriding parameter four types of lines were identified. (1) Lymphoblastoid cell lines, derived from normal and neoplastic hematopoietic tissue, were characterized by the wide morphologic flexibility of individual lymphoblastoid cells, constant association with Epstein-Barr virus (EBV), polyclonal derivation, differentiation for immunoglobulin production (secretion) and their diploids. (2) Lymphoma cell lines. This type of line was established at a high frequency from Burkitt's lymphoma and rarely from other types of lymphoma, but never from patients without malignancy or with non-lymphoma malignancies. Important characteristics were morphologic stereotypia within each line, monoclonal derivation, common but not obligatory association with EBV, variability in the expression of Ig synthesis (no production, or membrane bound Ig, or secretion) and aneuploidy. (3) Myeloma cell lines could only rarely be obtained from patients with myeloma. The basis for classification of these lines is their production of Ig identical to the myeloma protein in vitro. Other important distinguishing features were: plasma cell morphology, absence of EBV and aneuploidy. (4) The leukemia cell line (MOLT 4) was the only line with T-cell characteristics and was easily distinguished from the other types. Important characteristics were a typical surface ultrastructure, absence of EBV and absence of immunoglobulin production, Individual lymphoblastoid lines were in principle identical whereas each line of the other three types had its own characteristic profile. The phenotypic characteristics of the lymphoblastoid lines were very stable during prolonged serial cultivation. Only in a few cases were secondary chromosomal, functional or morphologic alterations noted. We conclude that EBV-carrying lymphoblastoid lines can be obtained from non-neoplastic precursor cells from healthy as well as from diseased individuals. Lymphoma, myeloma and leukemia lines are only obtained from the respective neoplastic tissue but generally at a low frequency. With the exception of Burkitt's lymphoma, malignant hematopoietic tissue and leukemia frequently give rise to established cell lines in vitro of the lymphoblastoid type rather than lines derived from the neoplastic cells;