Peripheral blood mononuclear cells (PBMC) from thirteen asthmatic children, and from normal control subjects, were pre-incubated with and without concanavalin A (con A), washed, and cultured with fresh allogenic PBMC from healthy donors. The con A pre-treated cells from fifteen of seventeen normal controls clearly suppressed the blast transformation response to con A by normal allogeneic PBMC. However, con A-generated suppressor activity was found in only seven of the asthmatic patients studied, most of whom could be classified as ‘short-term’ asthmatics. It is thus possible that either dysfunction or a reduction of the (con A)-inducible, T-suppressor cell subpopulation in peripheral blood is frequent among ‘long-term’ asthmatic patients. This may suggest that a different pathogenesis may be operating in early-onset, long-continued asthma, when compared with those investigated early in the course of asthma which has begun later in childhood.