We investigated structures resembling embryoid bodies (EBs), grown intraperitoneally in nude mice after the injection of xenografted human teratocarcinoma cells. Following in situ hybridization of paraffin sections containing these EB-like structures with either human or mouse total genomic DNA, two species-specific types of cell nuclei were localized. Tumor cells of human origin were found centrally but flattened normal mouse cells formed an outer coat. Thus these spheric structures are of bispecies origin and do not meet the definition of EBs. For a clear distinction from EBs and spheroids, we termed these structures chimeric spheroids.