To investigate the efficacy of Unsei-in on skin diseases such as Behçet's disease, palmoplantar pustulosis and psoriasis vulgaris, the rheological activity of leukocytes as manifested by adhesiveness, deformability and aggregability was studied in vitro using a micropore filtration method. Unsei-in inhibited the rheological activity of leukocytes at a statistical significance of p < 0.01 in normal individuals whose leukocytes had been stimulated with N-formyl-methionyl-leucyl-phenylalanine (FMLP). It did not, however, inhibit the activity of cells that had not been stimulated by FMLP. In contrast, in Behçet's disease, Unsei-in inhibited rheological activity of non-stimulated leukocytes (p < 0.01) but did not inhibit the rheological activity of FMLP-stimulated leukocytes. In a study using Ouren-gedoku-to and Shimotsu-to, the main constituents of Unsei-in, the latter inhibited the activity of FMLP-stimulated leukocytes in normal individuals (p < 0.02). These results indicate that Unsei-in has an inhibitory effect on rheologically-activated neutrophils, which are involved in the healing mechanisms of the above skin diseases, and it also has a satisfactory effect in the form of a mixture of Shimotsu-to and Ouren-gedoku-to.