The cytotoxic effect of methotrexate as evaluated by colony-forming activity of hemopoietic and tumor cells.

The response of hemopoietic and/or lymphosarcoma cells to methotrexate has been investigated. The proliferative capacity of hemopoietic tissue was measured by exogenous CFU assay. It was inhibited by a six times lower amount of MTX than the proliferative activity of lymphosarcoma cells as evaluated by liver-colony assay. The natural resistance of LS/BL lymphosarcoma cells to MTX might be attributed to the DHFR gene amplification resulting from DNA rearrangements appearing during malignant transformation.