The immunostimulating effect of Methisoprinol and KLP-602 on the development of organs of the immunological system, the reactivity of lymphocytes and selected biochemical parameters of allantoic fluid (the activity of ceruloplasmin and lysozyme) during embryogenesis was tested. A positive effect of these drugs, especially KLP-602, was found. KLP-602 positively affected the reactivity of lymphocytes and the development of immunocompetent organs of chicken embryos as well as it increased the activity of ceruloplasmin in allantoic fluid 6 hours after injection.