This project deals with a) the role of receptor cross-linking in B lymphocyte activation, and b) the role of T lymphocyte function in the switch from IgM to IgG antibody production: a) This problem is currently being approached by utilizing antibody-induced cross-linking of specific cell surface receptors through the mediation of an antigen bridge. These studies are in the preliminary phase. b) Previous work has indicated that the allogeneic effect, in the form of a graft vs. host reaction, probably does not induce a switch from IgM to IgG production but rather, enhances an ongoing response. Current work is aimed at further probing the role of T cells in IgG production by assessing the response to polysaccharides coupled to protein carriers, thereby utilizing protein specific T cell function rather than the non- specific allogeneic effect.