We describe a convenient infected cell hybridization assay for determining the infectious titer of a recombinant, replication-defective parvovirus. We previously generated recombinant derivatives of the autonomous parvovirus LuIII, transducing the luciferase reporter gene. Since luciferase expression is not readily detected at the single cell level, and since the recombinants cannot form plaques, we developed an alternative assay for infectious particles. Co-infection of NB324K cells with wild-type LuIII (multiplicity of infection ca. 5) and the recombinant virions allows amplification of the transducing DNA, which can be detected by hybridization with a probe for the reporter gene. Cell lysis and DNA transfer to a nylon membrane is performed in situ, in the culture dish, and hybridization is performed with a digoxigenin-labeled probe, using immunological detection. During this work we developed a conveniently modified plaque assay for wild-type LuIII that should be applicable to other lytic viruses. The modification employs a reduced volume of agarose overlay that is in turn overlaid with liquid medium, thus avoiding the need to maintain stocks of culture medium at higher than normal concentration.