The influence of sesquiterpene quinones and of a sesquiterpene hydroquinone, isolated from the sponge Smenospongia sp. on normal and tumour cells, was investigated. Most showed cytotoxic effects on L1210 leukemia cells. However, their activity on normal cells, such as murine spleen lymphocytes and human peripheral lymphocytes, revealed different behaviours: some of them inhibited, while other enhanced mitogen-stimulated lymphocyte proliferation. These biological studies revealed products modulating immune responses.