Aerial roots of two monopodial thick-leaved epiphytic orchids (Aranda and Vanda) are a poßible source of endogenous plant hormones (abscisic acid, cytokinins, indole-3-acetic acid). Using a monoclonal antibody-based immunoaßay, this study also revealed that the levels of plant hormones in root tips of Aranda could vary with root position along the stem, time and growth stages. The levels of plant hormones are usually higher in root tips than elsewhere in the roots. Higher levels of cytokinin (mainly isopentenyladenosine) were detected in root tips of flowering plants than in non-flowering plants of Aranda. Lateral aerial roots(s) could be induced by the removal of root tip. A poßible role for these rootderived plant hormones is discußed in relation to orchid growth and development.