The novel bisphosphonic acid ester derivatives represented by the following formula (1): €ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒY-Cy-(NH) m -(CH 2 ) n -C(X)(PO(OR 1 )(OR 2 )) 2 €ƒ€ƒ€ƒ€ƒ€ƒ(1) wherein each symbol is as defined in the DESCRIPTION, which has an amino group substituted by a heterocyclic group or a heterocyclic group containing a nitrogen atom, and the acid moiety is esterified with a POM group, an n-butanoyloxymethyl (BuOM) group and the like, exhibit a superior direct or indirect cytotoxicity effect on tumor cells and virus infected cells.
