Abstract: A merocyanine dye having the formula: ##STR1## wherein: R.sup.1 and R.sup.2 independently are H, alkyl, alkoxy, or aryl, andR.sup.7 and R.sup.8 may each be H with the proviso that a pair of R's selected from the pairs R.sup.1 and R.sup.2, R.sup.1 and R.sup.7, and R.sup.2 and R.sup.8 can together comprise the atoms necessary to form a fused aromatic ring on the benzene radical to which they are attached and only one of R.sup.1 and R.sup.2 can be H;R.sup.5 and R.sup.6 comprise alkyl of from 1-18 carbon atoms provided that the sum of the carbon atoms in R.sup.5 and R.sup.6 is at least 8;R.sub.9 is an alkylene group of 2-9 nuclear carbon and hetero atoms; andZ.sup.+ is a cation.This dye is useful in a method for inactivating viruses comprising contacting the viruses with the compound and exposing the resulting mixture to visible light to excite and inactivate the viruses. The compounds are also useful in the irradiation-induced inactivation of leukemia cells.