As a kind of cytotoxin extracted from the root tuber of Trichosanthes kirilowil Maxim (Cucurbitaceae), trichosanthin can selectively bind to and kill the placental trophoblastic cells, which leads to a number of biomedical applications including the inhibition of trophoblastic tumors. However, the stability of trichosanthin in living organism is still one of the problems hindering the effectiveness of its applications. In this study, laser light scattering has been used successfully to investigate the stability of trichosanthin in both deionized water and KSCN aqueous solution in terms of the hydrodynamic size distribution of the trichosanthin aggregates as a function of both time and the salt concentration. It is found that the size distribution is always a bimodal one. One peak corresponds to a single trichosanthin chain; the other corresponds to the trichosanthin aggregates, which have an average hydrodynamic radius of similar to 49 nm and are composed of similar to 127 trichosanthin molecules when C-KSCN is higher than 0.5 mol/L. This implies that there exists an equilibrium between the single trichosanthin chain and its aggregates [i.e., nT reversible arrow (T)(n)]. Our results also suggest that the aggregates are made of the loosely packed trichosanthin molecules and behave as flexible polymer chains in theta solvent.