High-dose methotrexate (MTX) cytotoxicity is decreased in MCF-7 breast cancer cells when the chemoendocrine agent tamoxifen (TAM) is given to cells 24 hours prior to MTX (early TAM). However, when breast cancer cells are exposed to TAM 24 hours after MTX (delayed TAM), MTX cytotoxicity is enhanced by TAM. The growth of cells exposed to 10 microM TAM and 10 microM MTX alone or in combination with early TAM plus MTX had the following order: TAM > TAM (early) + MTX > MTX. The percentages of control rates for TAM, MTX, and TAM (early) + MTX are 74.71 +/- 1.36%, 22.13 +/- 2.76%, and 38.17 +/- 2.75%, respectively. The inhibitory sequence from cells exposed to MTX + TAM (delayed TAM), MTX and TAM alone is MTX + TAM (delayed TAM) > MTX > TAM; and the percentages of control rates were 16.87 87% (MTX + TAM [delayed TAM]), 25.92 +/- 2.14% (MTX), and 54.08 +/- 14.79% (TAM). These studies suggest that: (a) the interactions between TAM and MTX are sequence-dependent; (b) TAM antagonizes the effect of MTX when TAM administration precedes MTX; and

(c) TAM enhances the effect of MTX when TAM administration follows MTX.