Intravascular red cell aggregation (IRCA) in the conjunctival microvessels was observed and photographed by capillary microscope in 110 patients with blood diseases, collagen diseases, diabetes mellitus and others. These findings were also investigated with special reference to erythrocyte sedimentation rate, plasma proteins and coagulation-fibrinolysis tests. The results were followings: 1) Significantly prominent IRCA was observed in collagen diseases, diabetes mellitus and dysproteinemia patients. 2) Patients with accerelated erythrocyte sedimentation rate showed prominent IRCA. 3) No significant difference of the mean value of total serum protein was observed between patients with prominent IRCA degree and those with low IRCA degree. 4) Significant low mean values of the serum albumin and A/G ratio were observed in patients with prominent IRCA. But no significant differences of the mean values of serum α(1)-, α(2)- and β-globulins respectively were observed between patients with prominent IRCA degree and those with low IRCA degree. 5) Significant hig hmean value of the serum γ-globulin was observed in patients with prominent IRCA. 6) SLE patient with positive cryoglobulin showed prominent IRCA in the nailfold and conjunctival microvessels, and the low flow state followed by the blood stasis, the perivascular edema and the Raynauds phenomenon were sequentially observed by local cooling. 7) No correlation was observed between IRCA and coombs test. 8) Significant elevation of the mean value of the plasma fibrinogen was observed in patients with prominent IRCA, but no correlation was observed between IRCA level and coagulation-fibrinolysis test.