Circulating lymphocyte subpopulations defined by anti-CD45 and other more common T-cell-specific monoclonal antibodies were analysed in 77 patients with multiple sclerosis and 38 healthy controls. A selective decrease of CD4+CD45+ cell percentages and absolute numbers in chronic-progressive patients was found; in 13 out of 26 patients this subpopulation was less than 11% CD4+CD45+ cells. Similarly, the whole CD45+ cell subset, as well as CD45+ cells expressed as percentages of CD4+ cells, were significantly reduced in chronic-progressive multiple sclerosis. CD4+CD45+ cells, commonly termed "inducer of suppression" T-lymphocytes, did not correlate with percentages or numbers of CD8+ cells. It is concluded that suppressor inducer T-cells act on the CD8+ subset function rather than reducing CD8+ cell numbers. Since CD4+CD45+ cells represent an early stage of lymphocyte maturation (naive T-cells), an under-representation of this subpopulation in active multiple sclerosis might reflect an increased conversion of naive cells into memory cells. This concept may be relevant for a better understanding of the disease pathogenesis.