Precipitation reactions regarded as `false positive' have been obtained to extracts of a variety of organic dusts. The antigen responsible for these reactions is an electrophoretically fast moving and negatively charged substance, previously observed in extracts of mouldy hay and other vegetable and organic dusts, and referred to as a `D line' antigen. This antigen has now been shown to have antigenic determinants in common with a β-teichoic acid prepared from Staphylococcus aureus. Absorption of the sera with N-acetyl-D-glucosamine had an inhibitory effect on `D line' and teichoic acid precipitin reactions.