B21 and B19 haplotypes of the chicken Major Histocompatilibity Complex (MHC) are associated with resistance and susceptibility to Marek's disease (MD), respectively. This thesis is an investigation of self and viral peptides associated with MHC-II molecules of these haplotypes. MHC class II (MHC-II) molecules were purified from chicken B21 and B19 homozygous B cells and associated peptides were eluted. Five B21 MHC-II associated peptides derived from two source proteins were identified. Thirty-eight B19 MHC-II associated peptides were identified. These were derived from 20 source proteins, one of which was the envelope glycoprotein of chicken syncytial virus (CSV). Binding assays verified that a CSV-derived peptide bound specifically to B 19 MHC-II molecules. All peptide sequences were employed to assign a peptide-binding motif to B19 MHC-II. To obtain a source of antigen presenting cells for future studies, a B19/B 19 B cell line was established which expressed the Marek's disease virus glycoprotein B (gB).