AbstractLettuce seed germination or lettuce root elongation after germination in water was inhibited by 5.7 × 10‐6Mindoleacetic acid (IAA) and designated other IAA derivatives. These IAA‐inhibited growth responses were reversed by 10‐5to 10‐6MCycocel, (2‐chloroethyl) trimethylammonium chloride, which alone was without effect. Only those Cycocel analogs, which have previously been shown to be active as plant growth retardants, were effective in reversing IAA inhibition of germination or root elongation. These results are consistent with the concept that Cycocel at low concentrations acts as an auxin antagonist. However. Cycocel did not reverse the inhibitory effects from indole‐3‐propionic acid or indole