Cells from 9 monolayer tissue cultures prepared from Hodgkin's disease tumors in the spleen were examined in the electron microscope. Three established culture lines (carried in vitro for greater than 3 years and passaged greater than 200 times) that contained aneuploid karyotypes were composed of oval cells with numerous interdigitating surface microvilli. The nuclei were complex and convoluted with multiple large nucleoli and dispersed chromatin. The cytoplasm contained lysosomes, microfilaments, a complex Golgi apparatus, nondilated rough endoplasmic reticulum, polyribosomes, fat, and glycogen. One Hodgkin's disease monolayer with aneuploid chromosomes examined from the 4th to 48th passage in culture was composed of larger cells with fewer microvilli and numerous multinuclear giant cells. Two monolayers derived from transplanted tumors in nude mice inoculated with Hodgkin's disease cultured cells were similar to the original cell lines. The ultrastructural features of these 6 cultures with aneuploid karyotypes differed from those of 3 monolayers which, although prepared from Hodgkin's disease splenic tumors, were composed of fibroblastic cells with diploid chromosomes. The aneuploid Hodgkin's disease cultures did not resemble 6 normal spleen, thymus, or lung monolayers, Raji lymphoblastoid suspension cultures, or Hela cells. Our electron microscopic studies indicate that adherent cells which replicate in some monolayer tissue cultures derived from Hodgkin's disease tumors are related to and possibly derived from neoplastic macrophages.