Experimental studies on the sensitizing capacity of naturally occurring benzoquinones, isolated from plants and woods have been carried out in guinea pigs of the Pirbright white strain. Seven compounds were available: primin, three dalbergiones, mansonia quinone (mansonone A), 2,6-dimethoxybenzoquinone and rapanone. With five of these substances (primin, mansonone A, three dalbergiones) guinea pigs could be sensitized. Primin, the allergen of Primula obconica Hance (primrose) proved to be the most effective one of all quinones tested in this and the preceding studies. As a similar but weaker sensitizer R-3, 4-dimethoxydalbergione from Machaerium scleroxylon Tul. (Pao ferro, Caviuna vermelha) could be identified. The results obtained with mansonone A, a sesquiterpenoid quinone from Mansonia altissima A. Chev. demonstrate that even naturally occurring orthoquinones are capable of inducing contact allergy. Allergic cross reactions could be obtained between all chemically related mansonones A-F. The results are in good accordance with the view that the sensitizing capacity of naturally occurring quinones depends on the fundamental quinoid structure and the length, position and configuration of the aliphatic side-chain.