Serum samples from 184 lepers are studied by means of solid phase ELISA with the semisynthetic antigen disaccharide-bovine albumin analogous of phenolic glycolipid I of Mycobacterium leprae. Patients are grouped according to the clinical presentation of the disease and course of time elapsed since the institution of therapy. Bacilloscopic examinations were made also in 116 patients who were positive at diagnosis. For serologic testing, absorbance values over 0,160 were considered positive. Results in multibacillary patients showed a gradual and significant decrease both of mean absorbance values and in the seropositive ratio in connection with the length of treatment. It was also found that phenolic anti glycolipid antibodies increased with the value of the bacteriologic index. This system is useful in monitoring the effectiveness of chemotherapy in multibacillary leprosy.