The effect of urinastatin (Miraclid) on the changes of peripheral lymphocyte subpopulations during the operation for malignancy was investigated. Ten urinastatin-treated patients and twelve non-treated patients undergoing surgery for gastric cancer under general anesthesia were studied. Peripheral lymphocyte subpopulations were measured before and during operation employing two color analysis utilizing two kinds of monoclonal antibody, such as Leu3a and Leu8, Leu2a and Leu15, or Leu4 and HLA-DR. With this analysis, it was possible to classify peripheral lymphocytes into helper T cells, inducer T cells, cytotoxic T cells. suppressor T cells and B cells. Decrease in inducer T cells, increase in suppressor T cells and decrease in B cells were observed in the non-urinastatin-treated group during the operation. Furthermore, decrease in OKT4/OKT8 (Leu3a/Leu2a) ratio was demonstrated. But these changes were inhibited in the urinastatin-treated group. This result suggests that the administration of urinastatin improves immunosuppressive state during operation.