Abstract In this study, the oils of Achillea talagonica and A. millefolium, which grow widely in Iran, have been analyzed by GC and GC/MS. The main constituents of the oil for A. talagonica were 1,8-cineole (9.7%) and camphor (21.9%), while the major components of the oil for A. millefolium were Î±-copaene (11.1%) and (E)-nerolidol (8.8%). Effects of these oils were studied on humoral immune responses in BALB/c mice. The oil isolated from Iranian A. millefolium ssp. millefolium possessed a high percentage of sesquiterpenes (55.4%) in which Î±-bisabolol was the main compound, while no proazulene or 1,8-cineole were detected. A high percentage of sesquiterpenes, presence of proazulene in A. millefolium (1%) and the lack of these compounds in A. talagonica could account for the different immunological effects of these plants.