Murine leukemic lymphoblasts L5178Y-R (LY-R) undergo conversion into their L5178Y-S (LY-S) variant as a result of prolonged (5 months to 4 years) cultivation in vitro. LY-R cells are highly tumorigenic in DBA/2 mice; resistant to X-rays [D0 (mean lethal dose [reciprocal of the slope of the linear portion of dose-survival curve] ) = 0.91 grays]; and sensitive to ultraviolet radiation (D0 = 0.7 J/sq m), short (up to 60 min) heat (43 degrees) treatment, and certain potential cancer drugs. LY-S cells are practically nontumorigenic in DBA/2 mice; sensitive to X-rays (D0 = 0.56 grays); and resistant to ultraviolet radiation (D0 = 5.5 J/sq m), short heat treatment, and the drugs. The differences in sensitivity of these two cell strains to physical and chemical agents are paralleled by differences in DNA repair efficiency. Although both strains can be cloned in soft agar, LY-S cells invariably show higher plating efficiencies. In vitro mean doubling times are 12 to 15 hr and approximately 10 hr for LY-R and LY-S cells, respectively. The actual loss of tumorigenicity and changes in radio- and photosensitivity associated with conversion of LY-R cells into LY-S cells occur within a short time. This indicates that these changes (and probably other phenotypic changes mentioned above) result from a single event or from several events occurring within a relatively short time elicited by in vitro culture conditions.