To add to our knowledge of collagen diseases, plasma fibronectin (FN) in patients with systemic lupus erythematosus (SLE) has been measured, and it was determined that the plasma FN value in those with SLE was 454 +/- 36 micrograms/ml, is significantly higher than the FN value in normal subjects (234 +/- 21 micrograms/ml) than that of patients with FN value of patients with active SLE was significantly higher (591 +/- 46 micrograms/ml) that of the patients with non-active SLE (287 +/- 31 micrograms/ml). The plasma FN value of SLE patients was also seen to be associated with the peripheral blood platelet count and with the dose level of the corticosteroid hormone administered to patients. In active SLE patients, it was similarly found that the plasma FN value had a significant correlation with the peripheral blood lymphocyte count and with the dose level of the corticosteroid hormone given to patients. Since the plasma FN value is known to be high in untreated SLE patients, it was felt that the increase of the FN value in SLE patients is not due to the effect of the corticosteroid but to the disease itself.