The notion of suppressor T cells has had a great impact in immunology and clinical medicine, A number of diseases, such as allergy and autoimmunity, are now considered to be caused by a relative lack of suppressor T cells, while other conditions, such as immunodeficiencies, are thought to be due to too many suppressor cells. Even fundamental immunological concepts, like immunological tolerance and the distinction between self and non-self, are often thought to be regulated by a delicate balance between suppressor cells and potentially autoreactive T and B cells. The concept of suppressor T cells has markedly affected the way immunologists interpret their findings. A depressed response is generally considered to be caused by suppression and implicitly by suppressor T cells. It is quite interesting to observe the changes that have occurred during the last 15 years: suppressor eells are now considered to be not only the essential, but also the only regulating elements of the immune system. The previous concept of powerful and specific regulating mechanisms, such as antibody feed-back inhibition ofthe immune response that efficiently regulates both T and B cells, now appears to be of little interest, I should like to analyse in general terms the plausibility of the eoneept that there is a separate T cell subpopulation with a suppressor function, which exists to control other cells of the immune system, I am not questioning the existence of suppressive phenomena or findings that T cells can mediate suppressive effects, but I am sceptical ofthe notion of suppressor Tcells as a separate subpopulation of T cells.