The essential oils from two Asteraceae species, Porophyllum ruderale (PR) and Conyza bonariensis (CB) were screened for anti-inflammatory activity in the mouse model of pleurisy induced by zymosan (500 microg/cavity) and lipopolysaccharide (LPS) (250 ng/cavity). The main monoterpene constituents of each oil, beta-myrcene (in PR) and limonene (in CB), were tested in the LPS-induced pleurisy model and assayed also for immunoregulatory activity by measurement of the inhibition of NO and production of the cytokines, gamma-interferon and IL-4. The oils, when administered orally, were able to inhibit the LPS-induced inflammation including cell migration; with a similar effect being observed for pure limonene. Pure beta-myrcene and limonene were also effective in inhibiting production of nitric oxide at doses below the cytotoxicity of these monoterpenes. A significant inhibition of gamma-interferon and IL-4 production by limonene and beta-myrcene was also observed.