A consistent immunological finding in patients with progressive multiple sclerosis is a loss of functional suppression. We have recently found decreases in suppressor inducer T cells in progressive multiple sclerosis as measured by two-color immunofluorescence using differentiation markers CD4 and 2H4. In the present study, we examined the relationship between functional suppression and circulating CD4+ 2H4+ T cells using a two-stage assay. (1) T cells were stimulated for 7 days with irradiated non-T cells (autologous mixed lymphocyte reaction [AMLR]) and harvested. It has previously been shown that suppressor T cells are generated during the course of the AMLR. (2) The AMLR-generated suppressor T cells were then incubated with mononuclear cells plus pokeweed mitogen, and immunoglobulin (Ig) synthesis was measured. There was less AMLR-induced suppression of IgG synthesis in patients with progressive multiple sclerosis as compared with normal subjects and patients with other neurological diseases. More importantly, there were significant correlations between decreases in circulating CD4+ 2H4+ cells and the AMLR (p = 0.009). Thus, the decreases in functional suppression and the decreases in the AMLR in multiple sclerosis appear tightly linked to CD4+ 2H4+ cells, and their measurement provides a means to monitor suppressor function phenotypically. Decreases in suppressor inducer T cells may in part explain immunoregulatory abnormalities observed in multiple sclerosis.