A series of novel 1'-methylxanthene-9-spiro-4'-piperidines has been prepared in the search for opiate analgesics with improved pharmacological properties. It has been found that introduction of a hydroxyl group into the 4-position of the xanthenespiropiperidine nucleus produces a potent mu-opiate agonist. The structure-activity relationship of the series has been explored by use of isosteric replacements of the phenolic hydroxyl group. Moreover, the effect of altering the conformation of the piperidine ring has been studied. It was interesting to note that, in compounds lacking the phenolic hydroxyl group, opiate activity could be produced by introduction of the (phenylamino)ethyl group instead of methyl at the 1'-position.