The Bursa cloacalis (bursa of Fabricius) and thymus are the central lymphoid organs of fowl and their presence is essential for the development of their adaptive immune function. The bursa and thymus control different immune responses (humoral or antibody-mediated and cell mediated or transplanted immunity) against a wide range of pathogens. Immunosuppression is a serious problem in commercial poultry flocks and estimation of economic impact is complicated and depends on the type of challenge, susceptibility of the flock, and focal environmental factors. In our experiment three groups of broiler chicken were fed on a complete diet for three weeks. Standard diet: control group-(C), contaminated diet: group A supplemented with 10 % and group B supplemented with 5% of the Galium aparine L. seeds. We observed clinical symptoms, body weight gain, haematological and biochemical values, pathological changes, humoral immunity, serology and histological changes of Bursa of Fabricii. We compared results of control group C with results of both experimental groups A and B. According to our results we confirmed the vasotoxic, hepatotoxic, nephrotoxic effects of Galium aparine L. seeds in diets for broiler chickens.