The present study has examined the effects of arborinine, an alkaloid obtained from Erthela bahiensis, a Brazilian plant popularly used as diuretic, antidiabetic, antithermic and expectorant, on the viability and function of immune system cells in vitro using a murine model. Rat spleen and thymus cells were cultured with 10 nM, 1 microM, 10 microM of the drug in the presence or absence of pokeweed (PWM), lipopolysaccharide (LPS), or concanavallin (ConA) mitogens. Cellular proliferation was analyzed by H3-thymidin uptake after 48 and 72 hr. Our results showed an inhibitory effect of arborinine on splenocytes proliferation under ConA or PWM stimulation and increased apoptosis on splenocytes and thymocytes stimulated with PWM in 24 hr. A decrease was observed on Interferon gamma (IFN-gamma) production by ConA- or LPS-stimulated splenocytes in 48 hr and 72 hr and ConA- or PWM-stimulated thymocytes in 72 hr. In contrast, an increase on lymphoproliferation was observed on LPS-stimulated splenocytes and ConA- or PWM-stimulated thymocytes in 48 hr. On this period, apoptosis decreased on LPS- or PWM-stimulated splenocytes and IFN-gamma production increased in PWM stimulated thymocytes. Arborinine also induced a decrease on Interleukin-10 production by splenocytes and thymocytes stimulated with ConA or PWM. There was no significant variation on the necrosis rate of the cells treated with arborinine or any change on their viability or function values in the absence of mitogenic stimulus.