The complement system may play an important role in the inflammatory reaction of psoriasis. While the classical pathway of the complement has been shown to be activated in psoriasis, there have been few reports on the involvement of the alternative pathway in the inflammatory reactions in psoriasis. Complement fragments, C4d and Bb, are released at the time of the classical and alternative pathway activation, respectively. The presence of the C4d or Bb fragment, therefore, denotes a preceding complement activation through the classical and/or alternative pathway. In the present study, we have measured the levels of C4d and Bb in psoriatic scale extracts using enzyme immunoassay techniques. The scales of these dermatoses contained higher levels of C4d and Bb detectable by enzyme immunoassay than those in the stratum corneum of noninflammatory skin. These results suggest that the alternative as well as the classical pathway of complement are activated in psoriatic lesional skin.