Novel process for the preparation of 11.alpha.-hydroxymethyl derivatives of natural (8R-) and racemic 9-keto-15.alpha.-hydroxy- and 9,15.alpha.-dihydroxy-prosta-5-cis,13-trans-dienoic and prost-13-transenoic acids, and the novel 15-methyl and ethyl substituted derivatives thereof, as well as the esters of the primary and secondary hydroxy groups and the non-toxic, pharmaceutically acceptable salts of the novel compounds.