A series of 5-amino-3-methyl-4-isoxazolecarboxylic acid semicarbazides and thiosemicarbazides (M1-M9) were obtained by reacting 5-amino-3-methyl-4-isoxazolecarboxylic acid hydrazide with isocyanate and isothiocyanate. In order to determine structure-activity relationships for the series of nine compounds (M1-M9) geometry optimization was carried out using quantum-chemical DFT calculations at B3LYP/6-31g* level. The compounds, administered to mice at 1 microg or 10 microg doses, four hours prior to immunization with sheep erythrocytes (SRBC), significantly suppressed the primary, humoral immune response in mice as measured by the number of antibody-forming cells (AFC) in the spleens. Structure/activity relationships, regarding the studied compounds, were discussed.