Prostaglandin analogs of the PGF.sub.2.sub..alpha., PGE.sub.2, PGF.sub.1.sub..alpha. and PGE.sub.1 series substituted at C-10.alpha. by a hydroxyl group, the derivatives of the PGF.sub.1.sub..alpha. and PGE.sub.1 series further substituted at C-5,6 by a methylene or dihalomethylene group, and the 10,11-ketals thereof and methods of preparing such compounds. 9.alpha.,15.alpha.-Dihydroxy-10.alpha.,11.alpha.-isopropylidenedioxyprosta-5,13-dienoic acid, 9-keto-10.alpha.,11.alpha.-isopropylidenedioxy-15.alpha.-hydroxyprosta-5,13-dienoic acid and 5,6-methylene-9.alpha.,15.alpha.-dihydroxy-10.alpha.,11.alpha.-isopropylidenedioxyprost-13-enoic acid are representative of the class. Also included are the corresponding esters, ethers, pharmaceutically acceptable salts and amides. These compounds possess prostaglandin-like activity and thus are useful in the treatment of mammals, where prostaglandins are indicated.