The nontransforming polyoma virus mutant NG-18, screened originally for its ability to grow in polyoma-transformed cells, has now been shown to grow in a variety of other cell types. Among cells found to be substantially permissive for the growth of NG-18 are: Phenotypic revertants of polyoma-transformed 3T3 cells, C-type RNA virus transformed and/or producing 3T3 cells, 3T3 cells that become transformed upon infection with leukemia virus, primary baby mouse kidney epithelial cells, and primary mouse embryo fibroblasts. Among cells found to be poor hosts for the growth of NG-18 are: Several 3T3 cell lines, SV40-transformed 3T3 cells, spontaneously transformed 3T3 cells, radiation- and chemical carcinogen-transformed 3T3 cells, and late passage mouse embryo fibroblasts. Similar results have been obtained using three other independently isolated host range mutants of the same type as NG-18. The permissive state thus does not require a cell-associated polyoma genome as the basis for complementation of growth for this class of host range mutant. In addition, the results demonstrate that cells can be permissive for the growth of such nontransforming virus mutants without themselves possessing properties commonly associated with transformation: Loss of density-dependent inhibition of growth, low serum requirement for growth, loss of anchorage dependence of growth, and lectin agglutinability. Implications of these findings are discussed in reference to possible mechanisms of action of the NG-18 gene in productive infection and transformation.