The activation of T helper lymphocytes involves the recognition of class II major histocompatibility complex antigens, which are dimeric glycoproteins (of subunit composition A alpha A beta or E alpha E beta) expressed on the surfaces of macrophages and B lymphocytes. One approach to understanding the relationship between the structure of these antigens and their functions in the immune response is to clone the genes that encode them, to obtain functional expression of the cloned genes transfected into an appropriate cell line, and then to see how those functions are affected in variant genes generated in vitro. We report here the expression in Iad-bearing B cells of an Ak beta gene, which confers on the transfected cells the capacity for both allostimulation and antigen-dependent activation of an I-Ak-restricted T-cell clone.