NALM‐6‐M1, one of eight leukemia cell lines cultured from the blood of a 19‐year‐old boy with non‐T, non‐B acute lymphoblastic leukemia (ALL) in relapse, was characterized. This cell line was found to be more than 90% cytoplasmic Immuno‐globulin positive (clg+) for both mu heavy and lambda light chains, but surface immunoglobulln (slg) and complement receptor (CR) negative. NALM‐6‐M1 was clg∼ for alpha, delta, and gamma heavy chains and kappa light chain. About 45% of the cells exhibited sheep erythrocyte receptors. Approximately 12% of cells were positive for Fc receptors. Approximately 90% of the cultured cells had a deleted long arm of chromosome 5 (5q−) and a marker Y chromosome. The remaining 10% of cells were found to have some additional chromosomal material on the long arm of chromosome 12, suggesting a partial translocation. No other karyotypic abnormalities were found. The cell line was found to react strongly with anti‐p23, 30, suggesting la‐like activity, but it only weakly stimulated normal lymphocytes in mixed leukocyte culture (MLC). The cells expressed HLA antigens. NALM‐6‐M1 failed to react with anti‐thymocyte serum (ATS), did not possess Epstein‐Barr membrane or nuclear antigens, nor did it exhibit phagocytosis for zymosan. NALM‐6‐M1 reacted positively with oil red O and Sudan black stains. On the basis of the clgM staining, NALM‐6‐M1 seems to be arrested at an early stage in B‐cell development and is considered to be of the pre‐B cell phenotype possessing a chromosomal abnormality.