The inhibition of the mitogenic activation of human peripheral blood lymphocyte (PBL) subpopulations by methylprednisolone (MP) was dependent on the mitogen used. Purified human T [thymus-derived] cells were more sensitive to the effects of MP than were B [bone marrow-derived] cells or PBL, especially when these cells were activated by pokeweed mitogen (PWM). MP did not function by inhibiting binding of mitogen to the cell surface. After being mitogenically activated, human lymphocytes were resistant to the effects of MP. These effects of MP were reversible. Monocytes did not provide a significant degree of protection to mitogenically activated human T cells incubated with MP. MP-induced inhibition of the mitogenic activation of human PBL may be a reflection of lymphocyte heterogeneity and the differential sensitivity of PBL to MP may be used to isolate functionally different subpopulations of these cells.