Germinal centres are histologically defined aggregates of blast cells that occur in B-cell areas of lymphoid tissues after antigenic stimulation. They are believed to be associated with the development of B-cell memory and plasma cell (especially secondary, IgG and IgA) responses. Recent studies of murine lymphoid tissues have defined cell-surface markers that distinguish germinal centre B cells from other mature B cells, permitting their identification and characterization in cell suspensions. Here we have used these markers to define and study germinal centre cells in lympho nodes, and have found that they constitute a unique population of B cells which (1) arises in response to antigenic stimulation, (2) contains nearly all of the demonstrably antigen-specific B cells in the stimulated organ, (3) bears surface IgM after primary stimulation and (4) as a population, demonstrates isotype switching to a predominant population, demonstrates isotype switching to a predominant surface IgG phenotype after secondary stimulation with specific surface IgG phenotype after secondary stimulation with specific antigen. These findings demonstrate that germinal centres are a major site of proliferation and differentiation of antigen-specific B cells in vivo, and suggest that the germinal centre microenvironment may have an important role in heavy chain class switching during B-cell responses.