(R)-(-)-Thiazolidine-4-carboxylic acid (I, l-thioproline), (4R)-(-)-2-thixo-4-thiazolidinecarboxylic acid (II), 2-thioxo-4-thiazolidinone (III, rhodanine), 4-oxo-2-thioxothiazolidine-3-acetic acid (IV, rhodanine-3-acetic acid) and thiazolidine (V) showed rather strong inhibitory activity on four kinds of plants examined at the concentration of 3.0×10^-4 M. In particular, four compounds except for V strongly inhibited the growth of roots of all the plants examined at the concentration of 1.0×10^-3 M. Furthermore, three compounds except for I and V inhibited the germination of the seeds of some plants at the concentration of 1.0×10^-3 M. The amount of chlorophyll in the cotyledons of Brassica rapa L. treated with 1.0×10^-3 M of I-V was decreased compared to that of the control group. Of the five compounds, III strongly inhibited the chlorophyll biosynthesis of this plant even at the concentration of 3.0×10^-4 M.