The activity of IgG, IgM and IgE antibodies to somatic antigen of Trichinella spiralis in the sera of patients with trichinellosis at various intervals after infection was examined by means of ELISA. Mathematical analysis of the dose-response curves was used. Elevated level of IgG and IgM antibodies of relatively high avidity and of rather low IgE avidity was documented. Amount or avidity ofIgG antibodies was found to be most useful for the diagnosis of trichinellosis (85% positive results in patients' sera). The isotype of IgM avidity constitutes a better diagnostic value than the amount of it (60% and 35% of positive results, respectively).