Size and charge of murine IgG-binding factors (IgG-BF) were determined. Four different sources were used to produce the factors: a) cells of a T cell hybrid (T2D4) constitutively secreting IgG-BF upon incubation in serum-free medium, b) T2D4 cells incubated with mouse monoclonal IgG1 antibody in order to induce in vitro the production of isotype-specific IgG1-BF, c) T2D4 cells induced in vivo by passage as ascites in nude mice and incubated in serum-free medium, and d) in vivo alloantigen-activated T cells (ATC) incubated in serum-free medium. IgG-BF were affinity purified on Sepharose beads coated with rabbit or mouse IgG and identified by their biologic activities, i.e., inhibition of in vitro secondary IgG antibody production to SRBC and inhibition of rosette formation between Fc gamma receptor-positive spleen cells and rabbit IgG-sensitized erythrocytes. IgG-BF produced by either of these cell sources was found to be heterogeneous in both size and charge. In each case, IgG-BF activities were recovered in three fractions of apparent Mr-74,000 to 78,000, 35,000 to 40,000, and 19,000 to 23,000-and in four fractions of pI-4.7 (or 5.3, depending on experimental conditions), 6.5, 7.7, and 8.4. Moreover, IgG-BF translated in vitro from T2D4 poly A RNA by using rabbit reticulocyte lysate exhibited the same heterogeneity. Thus, IgG-BF contain different proteins exerting similar biologic activities.