Five spontaneously derived BALB/c tumors and one 1-ethyl-1-nitrosourea induced BALB/c tumor (BALENLM 17), have been established in culture and characterized according to their surface markers. K46, X16C, L10A, and BALENLM 17 are IgM⁺, IgG^-, IgA^-, Ig⁺ (staining with polyvalent anti-Ig), Ia⁺, Fc receptor⁺ and complement receptor^-. A20 is IgM^-, IgG^-, IgA^-, Ig⁺, Ia⁺, Fc receptor⁺, and complement receptor^-. M12 is IgM^-, IgG^-, IgA^-, Ig±, Ia^-, Fc receptor⁺ and complement receptor^-. All these lines are Thy 1.2 antigen negative, do not phagocytize latex particles, and grow as stationary suspension cultures. Therefore these six BALB/c lymphoid lines are presumably of B cell origin at various stages of differentiation. All these lines except X16C are tumorigenic in mice within a short period of time and have relatively short generation times in the range of 18 to 26 hr. Karyotype studies show that L10A, K46, X16C, and BALENLM 17 are hyperdiploid to hypotetraploid. A20 and M12 have near diploid chromosome numbers.