Peripheral blood lymphocytes from 15 healthy subjects, selected on the basis of their HLA types, were cultured in vitro in the presence of PHA and of a number of drugs which bind adrenergic or dopaminergic receptors (dopamine, norepinephrine, chlorpromazine, haloperidol, propranolol and apomorphine). The results obtained are consistent with an interference between HLA-A1 and cross-reacting specificities and the effect explained by these drugs on the lymphocyte activation by PHA. The possibility is suggested that HLA-A1 and cross-reacting antigens interfere with the binding of such drugs to the cell membrane receptors.