In gram-negative bacteria, many important changes in gene expression and behavior are regulated in a popula tion density-dependent fashion by N-acyl homoserine lac tone (AHL) signal molecules. Exudates from pea (Pisum sativum) seedlings were found to contain several separable activities that mimicked AHL signals in well-characterized bacterial reporter strains, stimulating AHL-regulated be haviors in some strains while inhibiting such behaviors in others. The chemical nature of the active mimic com pounds is currently unknown, but all extracted differently into organic solvents than common bacterial AHLs. Various species of higher plants in addition to pea were found to secrete AHL mimic activities. The AHL signal-mimic compounds could prove to be important in determining the outcome of interactions between higher plants and a diver sity of pathogenic, symbiotic, and saprophytic bacteria.