Peroxidase activity and isoenzyme composition of cells and culture medium were analysed in a cell suspension culture of Haplopappus gracilis. The results revealed marked changes in the activity and isoenzyme patterns of peroxidase during the culture cycle. During the exponential growth phase the peroxidase activity of the cells increased and a considerable amount of peroxidase was released into the culture medium. Several peroxidase isoenzymes in the medium increased in activity continuously throughout the whole exponential growth phase while others decreased during the second half of this phase. The decrease of the latter was caused by diffusion of the isoenzymes back into the cell walls as well as by inhibitors. During the stationary phase of the cell cycle only small changes occurred in the peroxidase activity and isoenzyme patterns.