The role of accessory cells (antigen-presenting cells) in binding HBsAg in the response of human T cells to this antigen was studied. Antibodies to HBsAg of IgG class increased significantly the amount of HBsAg that was captured and internalized by accessory cells in vitro. On the other hand, antibodies to HBsAg of IgM class or the F(ab-)2 and Fab fragments of antibodies to HBsAg of IgG class did not modify the amount of HBsAg associated to these cells. HBsAg that was subjected to various denaturing treatments (acid, organic solvents, urea and heat) was compared for its capacity to react with antibody to HBsAg and stimulate the response of helper T lymphocytes. Results presented here indicate that HBsAg denatured by treatment with formic acid was captured by accessory cells and presented to the T cells much more efficiently than the native HBsAg. These results suggest that the response of helper T lymphocytes to some antigens such as HBsAg can be affected greatly by the presence of antibodies or the antigens- conformation.