Abstract:Idiotypic restrictions are demonstrated in vitro for the cooperation between T and B lymphocytes with specificity for Group A streptococcal carbohydrate. T helper cells which have been primed in vivo with anti‐idiotypic antibodies to the A5A idiotype and which are therefore essentially A5A idiotype‐positive, cooperate only with A5A idiotype‐positive B cells, even when mixtures of A5A idiotype‐positive and A5A idiotype‐negative B cells are present. Essentially A5A idiotype‐negative T helper cells that have been primed in vivo with Group A streptococcal vaccine after in vivo suppression with anti‐A5A idiotypic antibody are unable to cooperate with B cells which have been primed with anti‐A5A idiotype antibody and which are therefore essentially A5A idiotype‐positive. Mixtures of A5A idiotype‐negative and A5A idiotype‐positive T cells cooperate with both A5A idiotype‐negative and A5A idiotype‐positive B cells.Idiotypic restrictions could not be demonstrated for T and B cells recognizing carrier and hapten determinants, respectively, in experiments in which the cooperation of genetically VH‐identical T and B cells was compared to the cooperation of genetically VH‐different T and B cells. The data are discussed with respect to various models for the communication between T and B cells. It is proposed that for successful T‐B cooperation, ordinarily two types of T helper cells are required, one recognizing the antigen and the other recognizing the idiotype of the B cell.