The immunomodulator properties of two species of halophilic Archaebacteria, Halobacterium saccharovorum and Halococcus rnorrhuae, were analysed by the study of lymphocyte activation. Two methods were used to detect activation in lymphocytes, namely incorporation of the radioactive nucleotide [3H]-thymidine, and CD25 expression. H. morrhuae had a stimulatory effect on human lymphocytes, but this action was observed only with the [3H]-thymidine uptake method, whereas H. saccharovorum produced no immunomodulator effect.