It has been shown that cells transformed by known oncogenes could be reverted to an untransformed phenotype by the antibiotic Azatyrosine (AzTyr). In order to evaluate the reverting effect of AzTyr on five spontaneously transformed FR3T3C cell clones, we performed three assays: soft agar clonability, tumorigenicity in nude mice and susceptibility to killing by the parvovirus minute virus of mice (MVMp). In contrast to untransformed cells, transformed or tumorigenic cells are permissive for the lytic replication of MVMp and are killed. Our results demonstrate that although the cell populations that emerged after AzTyr treatment of FR3T3C clones had different phenotypes (two were untransformed and two had an altered transformed phenotype), they all behaved like untransformed cells, as judged from their resistance to MVMp infection. Our results demonstrate that susceptibility to MVMp is a valuable way to monitor the reversion.