Dynamic changes of T and B lymphocyte subsets, and immunoglobulin (Ig) synthesis in cultured B lymphocytes were studied in accordance with the clinical activity in 29 adult patients with minimal change nephrotic syndrome (MCNS) . The subsets (T3, T4, T8, sBγ, sBα, sBμμ, and sBε) were identified by detection of cell surface markers such as OKT3, OKT 4, OKT8, and γ, α, μ and s chain of Ig respectively. Cytoplasmic Ig positive cells (cB) were studied after culture of peripheral lymphocytes for 5 days. In the nephrotic phase (n=10), T lymphocyte subsets remained unchanged, but sB (sBγ, sBα, sBμ, sBε) and spontaneous cB (cBγ, cBα, cBε) were increased. However, cB induced by pokeweed mitogen (PWM a 10μg/ml) did not differ significantly from the control levels, and the PWM stimulation index (St. I. = cB with PWM/cB without PWM) in the IgG producing system was decreased (p<0.005) . In the remitting phase by steroid therapy (n=10), T3, T4 and T4/T8 (helper/suppresser) ratio revealed a significant reduction in an accompa-niment with a tendency of normalization of B lymphocyte subsets, However, spontaneous and PWM induced cB were increased no more than the control value, resulting in a significant reduction of cB/sB ratio. In the remitted phase requiring no steroid therapy (n=9), all of the parameters were normalized except for the PWM St. I. in the IgG producing system, which was decreased (p<0.025) . Autologous plasma of nephrotic phase significantly reduced cB with or without PWM, but remitted plasma did not. These results suggest that in the nephrotic phase of MCNS B lymphocyte subsets are already activated in vivo, but their proliferation or maturation from sB to cB is suppressed.