Sister chromatid exchange in methotrexate resistant and sensitive C3H10T1/2 mouse cells.

Sister chromatid exchange (SCE) induction by methotrexate (MTX) was analyzed in C3H10T1/2 clone 8 mouse cells and in two MTX-resistant subclones with numerous double minute chromosomes (DM) present in the majority of cells. Significantly higher SCE levels were found, as expected, in sensitive cells after treatments with 10(-2) or 10(-5) M MTX but not in resistant cells permanently growing in the presence of a high concentration of MTX (2 x 10(-3) M) and characterized by a markedly lower cell cycle replication index (R.I.), i.e. in conditions that are known to otherwise favour SCE induction. These observations suggest, for the MTX-resistant cells under study, the existence of conditions limiting SCE formation.