The effect of an extract from Melissa officinalis on immune response in mice was analysed using the cytotoxicity test in three dilutions (undiluted extract and extract diluted 10 and 100 times) and hemagglutination and rosette tests with various routes of administration (oral and subcutaneous). The immunostimulating activity of the extract was compared with that of a synthetic compound--levamisole, which influence on the immune system is well known. The present results confirm the effect of water extracts from leaves of Melissa on the immune system, in both humoral and cellular response.