In the course of our research project on plant growth regulators among microbial metabolites, we isolated two macrolide plant growth inhibitors, kaimonolide A (KA) C_<40>H_<68>O_<12> and kaimonolide B (KB) C_<35>H_<58>O_<10>, from Streptomyces No. 4155 isolated from a soil sample collected near Mt. Kaimon in Kagoshima Prefecture. Mainly by ^1H-, ^<13>C-NMR, ^1H-^1H, ^1H-^<13>C, ^1H-^<13>C long range COSY NMR and HOHAHA, the structure of the dehydrate of KA was elucidated and consequently KA was determined to be a 22-membered macrolide. Similarly the structure of KB, an 18-membered macrolide, was determined by means of the analysis of nmr spectra. In a bioassay using lettuce seedlings (Lactuca sativa L., cv. Great Lakes 366), KA inhibited root elongation by about 50% even at 0.2 ppm, and gave rise to necrosis at 0.5 ppm. On the other hand KB inhibited root elongation by about 50% at 10 ppm.