Cell suspension cultures of Solanum carolinense L. were treated with 2.8 x 10 -8 M chlorsulfuron, chlorsulfuron plus a 1:1 mixture of isoleucine and valine (each at 10 -4 M), isoleucine and valine alone, or left untreated for a period 10 days. In chlorsulfuron treated cultures, cell division was almost completely inhibited while the addition of isoleucine/valine reversed the inhibitory effects of chlorsulfuron on cell proliferation. Chlorsulfuron had no effect on the proportion of tracheary elements formed per total cell number in culture. Chlorsulfuron is known to arrest cell cycle progression in G1 and G2 by inhibiting the biosynthesis of isoleucine and valine. These amino acids are therefore implicated in the control of cell cycle activity but not cytodifferentiation. The results of this study also provide further evidence that cell division is not a requirement for the differentiation of tracheary elements in vitro .