The luteoviruses barley yellow dwarf virus (BYDV-PAV and BYDV-RPV), bean leaf roll virus (BLRV) beet western yellows virus (BWYV), carrot red leaf virus, potato leaf roll virus, and soybean dwarf virus (SDV) were compared by hybridization with random cDNA probes and serologically with polyclonal antisera. For hybridizations, filters had each RNA blotted in duplicate dots at 10 ng/dot. Random-primed cDNA probes were prepared from 300 ng of each RNA and used to probe filters at three levels of stringency. Homologies were observed between BLRV and SDV and between BWYV and BYDV-RPV at the lowest level of stringency (Tm-38). In double-antibody sandwich enzyme-linked immunosorbent assay using polyclonal antisera, two-way relationships were observed between BWYV and BYDV-RPV. In indirect enzyme-linked immunosorbent assay, where purified virus denatured in pH 9.6 carbonate buffer was coated directly onto microtiter plates, relationships between members of the luteoviruses were much more extensive. BLRV, BWYV, BYDV-PAV, carrot red leaf virus, potato leaf roll virus, and SDV antisera reacted with each of the six luteoviruses tested in the indirect test, while the BYDV-RPV antiserum reacted only with BYDV-RPV, BWYV, and BLRV. BWYV and BYDV-RPV are closely related in both tests and should be considered strains of one virus.