Pretreatment with berberine stimulates IL-12 production in mouse macrophages and dendritic cells. Mouse macrophages (a) or dendritic cells (b) were pretreated with berberine (0·1, 0·5, and 1·0 μg/ml) or untreated. After 6 hr, the cells were washed and stimulated in vitro with medium alone, or with either LPS (5 μg/ml) or CD40L–CHO cells for 48 hr. The culture supernatants were harvested, and the levels of IL-12 were evaluated by ELISA. The results are presented as the means ± SEM (n = 3). *P < 0·01, relative to groups which were not pretreated with berberine. **P < 0·05, relative to groups which were not pretreated with berberine.