Abstract We have recently described a potential antineoplastic effect of prostaglandin D 2 (PGD 2) ( Fukushima, M., Kato, T., Ueda, R., Ota, K., Narumiya, S., and Hayaishi, O. (1982) Biochem. Biophys. Res. Commun. 105 , 956–964). In the present study three analogues of PGD 2 were examined for their antineoplastic and other biological activities. 9-Deoxy-Δ 9-PGD 2, a dehydrate of PGD 2, had about 3 times stronger growth inhibitory effect than PGD 2 on L1210 leukemia cultured cells. The IC 50 value of this compound was 0.5–1.0 μg/ml (2μ M ). In contrast to PGD 2, this compound lacked a contracting activity on colonic smooth muscle and caused less diarrhea. The antiaggregatory action of the compound on blood platelet was about 10 times weaker than PGD 2. The growth inhibitory activity was also observed with cyclopentenone itself; the IC 50 value was about 100 μM. On the other hand, BW245C which had the antiaggregatory activity comparable with PGD 2 showed no growth inhibition. These results suggest that cyclopentenone structure is an essential moiety of prostaglandin derivaties for cell growth inhibition and that the growth inhibitory activity could be dissociated from other biological activities of prostaglandins.