In a former communication (Cook, Dodds, Hewett, and Lawson, 1934) we recorded the oestrogenic activity of a number of polycyclic aromatic and hydroaromatic compounds. The most active substances were found among a series of diols related to the carcinogenic hydro carbon, 1 :2 :5 : 6-dibenzanthracene. We have now extended our investi gation to seven new members of this series and have also examined a variety of diols of analogous structure, but containing other ring systems. Table I summarizes the results obtained in tests for oestrogenic activity carried out with 9: 10-dihydroxy-9: 10-dialkyl-9: 10-dihydro-1: 2: 5: 6-dibenzanthracenes. In none of the new compounds has the activity exceeded that of the most potent compound previously described, namely, the di-/z-propyl derivative in this series. Among the straight-chain dialkyl compounds this di-n-propyl compound represents a peak of activity in a series of which the first (methyl) and higher (//-amyl, nhexyl) members are inactive. Two branched-chain compounds propyl, wo-butyl) are fairly highly active, but show diminished activity by comparison with the corresponding straight-chain isomerides. This is particularly marked for the /su-propyl compound, which is about ten times less active than the //-propyl compound. In view of the high order of activity of the /7-propyl compound, it is remarkable that the allyl on May 10, 2018 http://rspb.royalsocietypublishing.org/ Downloaded from