ELISA, cDNA dot blot hybridization and transmission by vector aphids were used to investigate the occurrence and degree of cross-protection produced in oat plants by virus isolates representing five strains or serotypes of barley yellow dwarf virus, namely PAV, MAV, SGV, RPV and RMV. Generally, the degree of cross-protection was positively correlated with the serological relatedness between the isolates. A high degree of cross-protection occurred between NY-MAV and MAV-PS1, two isolates of the MAV serotype; cross-protection was moderate between MAV-PS1 and either P-PAV (a Purdue isolate of the PAV serotype) or NY-SGV; cross-protection between P-PAV and NY-SGV was low. Cross-protection did not occur in other paired inoculations and did not persist in some plants, the challenge virus eventually becoming detectable. The persistence of cross-protection depended on the interval between inoculations with protecting and challenge viruses; longer inoculation intervals enhanced the persistence of cross-protection. Results obtained by ELISA and dot blot hybridization were usually consistent, indicating that cross-protection affected both viral capsid and RNA synthesis.