The objective of these studies is to analyze the fine structure of the antibody combining sites which bind a defined, site filling antigenic determinant of six or seven Alpha(1 Yield 6)-linked glucose residues. Changes in the amino acid sequence of the variable regions of heavy and light chains of mouse hybridoma and myeloma anti-Alpha(1 Yield 6) dextrans will be correlated with changes in combining site size, affinity and idiotypic determinants. The relative contributions of CDR1, 2, and 3 and of VH and VL to the combining site will be assessed. The origins of antibody diversity (e.g., somatic or germ-line) to this well defined, T independent antigen will be determined. Chain recombination and in vitro mutagenesis with expression of the mutagenized chains by gene transfection will be used to further define the contribution of specific amino acid residues to the combining specificity.