The availability of an ultramicroanalitic system (SUMA) and specie-specific antigen of M. leprae obtained by chemical synthesis, have made possible the standardization and validation of an ultramicroELISA assay for detecting specific human IgM antibodies to this mycobacterium. The specificity of this test to demonstrate the infection with M. leprae was corroborated through a screening of 433 blood bank serum samples and other 265 from diferent groups (100, control group, 50 tuberculosis patients, 65 leprosy patients, 50 from household). The results obtained in the aditional study of 140 household sero showed a high correlation (r = 0.98) with the conventional microELISA method. The use of SUMA allows saving reagents and time since sample handling, plate reading, print out and storing the data are computer assisted.