Eight examples of nephroblastoma induced transplacentally in the partially inbred IIIVO/J strain of rabbit by a single intraperitoneal (60 mg/kg) dose of N-ethylnitrosourea (ENU) were studied by transmission electron-microscopy. At the light-microscopic level the ENU-induced tumors displayed the complex array of histotypic components described in a previous report (Hard GC, Fox RR: Histologic characterization of renal tumors (nephroblastomas) induced transplacentally in IIIVO/J and WH/J rabbits by N-ethylnitrosourea. Am J Pathol 1983, 113:8-18) namely, blastema, tubular profiles, "reniform" islands, glomeruloid bodies, squamoid foci, fascicles of mesenchymelike tumor cells, and an increasing fibrocollagenous stroma. Ultrastructurally, blastemalike cells were undifferentiated forms resembling metanephric blastema. Tubular profiles of varying development were typified by very prominent apical junctional complexes and a basal lamina, but no organized brush-border. "Reniform" islands were groups of more maturely formed tubules associated with the production of an interstitial matrix consisting almost solely of multilamellar basement membrane. Glomeruloid bodies were invaginations of small podocytelike cells, with a profusion of thin cytoplasmic processes resembling foot processes, and internal, homogeneous areas of basement membrane continuous with that surrounding the entire structure. As such, these structures were consistent with primitive attempts at glomerular differentiation but without vascular or mesangial elements. The squamoid foci were representative of true squamous differentiation in comprising cells filled with intermediate filaments and interconnected by lateral interdigitations and multiple, well-formed desmosomes. Spindle-shaped tumor cells disposed in fascicles, which could have been interpreted as bipotential differentiation into secondary mesenchyme at the histologic level, differed from the blastemal cell type only in shape. Ultrastructurally, the fascicles, in most cases, consisted of thin, basement-membrane-invested ribbons of attenuated cells enclosing small lumens sealed by intercellular junctions, suggestive of abortive tubule formation. Tumor cells disposed in fascicles therefore conformed to the same epithelial lineage as the other neoplastic components of these tumors. In contrast, cellular elements of the fibrocollagenous stroma--namely, fibroblasts, vascular endothelial cells, and scattered mononuclear inflammatory cells--could be clearly discriminated from the various tumor cell components on morphologic grounds, constituting a host reaction.(ABSTRACT TRUNCATED AT 400 WORDS)