The histology of five implantation tumors induced in rats by the deposition of cultured cell lines derived from dimethylnitrosamine (DMN)-treated rats is described and compared with the morphology of the predominant kidney neoplasm induced in vivo by a single high dose of DMN. The cell lines leading to growth upon implantation were long-established, continuously growing cultures obtained either from a DMN-induced renal mesenchymal tumor or from rats treated shortly before with a carcinogenic dose of DMN. The latter cultures had expressed morphological transformation at subcultures 5 or 6. All of the implantation tumors were of mesenchymal type, comprising variously a range of cell forms including fibroblast-like spindle cells, smooth muscle fibers, and "giant" cells, which resembled common aspects of the parent mesenchymal tumors induced in the rat kidney by DMN. Deposition of cells intrarenally illustrated the survival of remnants of preexisting nephrons as epithelial profiles scattered through the proliferating malignant tissue, a feature most characteristic of the parent tumor. The results confirmed the malignant nature of the various cell lines tested, in keeping with their altered behavior in vitro, and they were consistent also with the premise that the in vivo-in vitro system is selecting cells in culture that represent the same target population from which the renal mesenchymal tumors are derived in vivo.