Novel 18 -, 19 - and 20 -hydroxy-prostaglandin derivatives of the formula I ##STR1## wherein the dotted line in the position 8-12 indicates the optional presence of a double bond, the waved lines in position 15 indicate that the hydroxyl group and the group R.sub.4 are either in .alpha.- or .beta.-position and Z represents a --CH.sub.2 CH.sub.2 -- or a cis --CH.dbd.CH-- group, and wherein R represents one of the groups: ##STR2## (wherein the waved lines indicate that the hydroxyl groups are either in .alpha.- or .beta.-position and R.sub.1 represents a hydrogen atom, a methyl or ethyl group), R.sub.2 represents either an oxygen atom or a hydrogen atom and an .alpha.- or .beta.-hydroxyl group, R.sub.3 represents a hydrogen atom or a hydroxyl group and R.sub.4 represents a hydrogen atom or a methyl group, with the proviso that when simultaneously, R.sub.1, R.sub.3 and R.sub.4 each represents a hydrogen atom, R.sub.2 represents an oxygen atom, a double bond is in 8-12 position and the 15-hydroxyl group is in position .alpha., R does not represent the group (b), but that when in addition to these conditions, Z represents a cis --CH.dbd.CH-- group and the 8-12 position is saturated, R either represents the groups (b) or (c); and the pharmaceutically acceptable salts and esters thereof, novel process for their preparation by selective microbiological hydroxylation of compounds of formula II ##STR3## wherein the dotted line in the position 10-11 indicates the optional presence of a double bond in case the 8-12 position is saturated and the other symbols are as defined hereinabove, by means of microorganisms of the Division of Eumycota or, as far as the introduction of a hydroxyl group in the 18- or 19-position is concerned, of the Family of Streptomycetaceae, and, if desired, conversion of the 18- 19- and 20-hydroxy-prostaglandin derivatives thus obtained into pharmaceutically acceptable salts and esters thereof, and pharmaceutical compositions containing at least one of the novel hydroxy-prostaglandin derivatives of formula I.