A new polymorphism of the complement factor C3 in human plasma was demonstrated by isotachophoresis in agarose gels followed by immunodetection with rabbit anti-human C3c and C3d immunoglobulins. Four bands were detected in the immunoprint of freshly drawn EDTA-plasma, which were C3s1, C3s2, C3f1 and C3f2. At least four additional C3 components in Mg2+ -zymosan activated plasma were present, which were C3b1 to C3b4. The different forms of C3 in frozen and thawed heparin-plasma from 20 patients with psoriasis and 20 healthy individuals were studied from the immunoprint. The total content of C3 components was 29% greater in the patients with psoriasis than controls. The major difference was in the C3b components which were increased by 46%. In psoriatic patients, the two slow C3 components C3s1 and C3s2 were increased by 24 and 56% respectively, when compared with controls. The two fast C3 components C3f1 and C3f2 were decreased to 29 and 37%. The results suggest a direct involvement of the complement factor C3 in psoriasis.