By using single radial immunodiffusion we measured the plasma levels of four complement modulating proteins, i.e., C1 inhibitor, C4-binding protein, factor H and factor I in 19 psoriatic patients in comparison with those of healthy controls. Except for C1 inhibitor which was only marginally elevated, they were found to be significantly increased in psoriatic patients. When psoriatic patients were classified into subgroups based on the clinical severity, the levels of factor H and those of factor I showed a close positive correlation with the activity and extent of the skin lesions, whereas such clear relationship could not be found with C1 inactivator or with C4-binding protein. These results offer additional support for the hypothesis that the complement system is involved in psoriasis.