The presence of circulating immunocomplexes (CIC) was evaluated in several collagen diseases and in a control group of 100 healthy individuals. Three methods were used for their detection: binding to C1q in solid phase, binding to conglutinin in solid phase, and measurement of the serum capacity to solubilize an experimental immunocomplex. In the group of patients with systemic lupus erythematosus (SLE) significant differences were found for the three techniques (p less than 0.001) and also for activity (p less than 0.001). The most sensitive method was binding to C1q. The sensitivity of the three techniques for CIC was very low in the group of patients with systemic sclerosis, and the highest rate of positive results was found with binding to C1q (10%). In the group with hypersensitivity vasculitis and polyarteritis nodosa CIC were found in 71% of cases, more than one method being positive in 50%. The highest sensitivity was obtained with the conglutinin method (48%). In patients with temporal arteritis, significant differences were only found for conglutinin binding method (p less than 0.001), with low rates of positivity.