Methotrexate susceptibility in Leishmania cells. (a) Leishmania cells were grown in M199 medium and their growth was monitored at 72 hours by measuring their OD600 nm with varying concentrations of MTX. White circles, L. major wild-type cell; black circles, L. major MTX60.4; white squares, L. infantum wild-type cells; black squares, L. infantum MTX20.5. (b) The mutant L. major MTX60.4 was grown in the absence of drug for 5, 12, 25, 30 and 42 passages. The average of triplicate measurements is shown.