The ester derivatives of 4-chloroindole-3-acetic acid (4-Cl-IAA) with L-lactic acid and L-lactic acid esters were synthesized by two schemes from 4-Cl-IAA and L-lactic acid or its esters. Their biological activities were determined by three bioassays (hypocotyl growth inhibition in Chinese cabbage (Brassica pekinensis cv. Kinshu), hypocotyl swelling and lateral root formation in black gram (Vigna mungo (L.) Hepper), and adventitious root formation in black gram cuttings). All the L-lactic acid ester derivatives had strong activities in the three bioassays, but their activities were somewhat weaker than those of 4-Cl-IAA in the first two bioassays. Root formation activity of some esters in cuttings was higher than, or the same as, that of 4-Cl-IAA. At 1 X 10-5 M, roots induced by the esters were about 2 to 4 times those of 4-(3-indole)butyric acid (IBA). Unexpectedly, the free lactic acid derivative had the weakest activity of all the lactic acid derivatives tested in all bioassays. © Pesticide Science Sociey of Japan.