Infectious bursal disease virus (IBDV) strains have been identified and placed into molecular groups by a reverse transcriptase (RT)/polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) assay. The predicted amino acid sequences corresponding to the region of the genome examined by RFLP were determined and compared for 14 IBDV strains from different molecular groups and 11 IBDV strains that were identified in molecular group 6. Among the viruses within molecular group 6, 13 amino acid positions had mutations, and among the viruses in different molecular groups, 27 amino acid positions had mutations. In addition to having more mutations, viruses compared from different molecular groups also had mutations at key positions that were previously reported to be important for the formation of neutralizing epitopes. Three of these IBDV strains with unique RFLP patterns were used to challenge 1-wk-old broiler chickens with maternal immunity to IBDV. One of these viruses, T1, broke through this maternal immunity as evidenced by detection of the virus by RT/PCR-RFLP and production of an active virus neutralizing antibody response to classic and variant IBDV strains. Unique amino acid mutations in the T1 virus that may have contributed to its ability to break through this maternal immunity were observed at amino acids 318 and 322. The results indicate that RFLP profiles and nucleotide sequences can be used to predict the relative similarities and differences among IBDV strains, but determining the actual antigenic differences among viruses requires testing in vivo.