It has been shown in our previous study that the lymphoproliferation to allo- or soluble antigens could be inhibited when T cells were cocultured with antigen presenting cells (APCs) pulsed with Trichosanthin (Tk), a plant protein purified from a Chinese medicinal herb Trichosanthes kirilowii Maximovich. In this paper, data are presented dealing with the mechanism by which the Tk functioned as a down modulator. APCs of human peripheral blood were first treated with one of inhibitors of antigen processing (chloroquine, leupeptin or cycloheximide), followed by pulsed with Tk, and then added into a T cell culture with stimulators PMA and A 23187. The suppression of lymphoproliferation was observed to be obviously diminished or totally disappeared. In contrast, when CyA was used to replace the Tk for pulse-treatment of APCs that had received the same pretreatment with the inhibitors, no significant change was detected for the suppression, suggesting that the Tk might use a different pathway to induce hyporeactivity and that the pathway was concerned in APC's function of antigen presentation. This view obtained support from our immuno-histochemical examination of the Tk-pulsed APCs. By preparing colloidal gold-labelled Tk (Tk-G particle), we were able to show that the Tk-G, when incubated with APCs and T cells at 37 degrees C for two hours and washed, was visualized under the electronic microscope to be bound to APCs', instead of T cells', membrane surface and internalized cells' into endosomes. And then the Tk-G particles were further identified within lysosomes. In this way, the Tk molecules were subjected to be processed as an external antigen and might be presented to T cells to activate certain T subsets that in turn mediated the immunosuppression. This course was capable of, as expected, being interrupted by antigen processing inhibitor, especially by chloroquine.