The ability of the Chelidonium majus L. alkaloid derivative Ukrain (UK) to inhibit ovalbumin-induced sensitization was tested in BALB/c and F1(BALB/c x C57BL/6J) mice. UK introduced into the mice in the mixture with antigen (ovalbumin) and adjuvant (alum) inhibited the sensitization of mice, reflected in lower anti-OA IgE antibody response and decreased antigen-induced histamine release from mast cells isolated from peritoneal cavities of sensitized mice. The effect of UK on the antigenicity of ovalbumin (OA) in anaphylaxis was tested in heterologous passive cutaneous anaphylactic (PCA) reaction on rats. The results show that the OA prepared in the mixture with UK had a decreased ability to react with anti-OA IgE antibodies raised against native OA in mice and fixed on the surface of rat mast cells in heterologous PCA reactions. The results suggest that UK pretreatment of OA may affect its antigenic property and the ability to react with anti-OA IgE antibodies raised against the native IgE molecules.