B6AF1 anti-B10.D2 ascites fluid was fractionated by molecular sieving. From the 7S fraction, IgG subclasses were isolated by affinity chromatography with specific antisera against mouse immunoglobulin subclasses coupled to CNRr-activated Sepharose 4B. Thus, 7S IgG1, 7S IgG2, 7S IgG2a, and 7S IgG2b were obtained which were completely pure by criteria of immunodiffusion and immunoelectrophoresis. In the in vitro tests the 7S IgG1 completely lacked cytotoxic activity. All the other subclasses were able to induce lysis of B10.D2 cells in the presence of rabbit complement. All subclasses were tested for their capacity to induce enhancement and hyperacute destruction of B10.DI skin grafts in B6AF1 recipient mice, and for the dose-dependency of these effects. 7S IgG1, although showing clearcut enhancing activity, was completely inactive in inducing hyperacute destruction of the grafts, even if high doses were administered. By contrast, 7S IgG2 (IgG2a as well as IgG2b) was not only able to induce enhancement but also hyperacute destruction of the frafts. The results obtained with 7S IgG2 demonstrate that opposite biologic activities can be present within a single subclass.