AbstractThe determination of DNA with 3,5‐diaminobenzoic acid (DABA) following a DOEBNER‐MILLER‐quinaldine synthesis (PEREZ 1950), the reaction of which is specific for deoxyribose, is rarely known and used.Hydrolysis of DNA and condensation of liberated deoxyribose with DABA (prepared via ethyl‐3,5‐dinitrobenzoate) can be performed in an one‐step‐reaction resulting in a stable and sensitive fluorescence with favourable excitation and emission conditions. In dependence on the fluoro‐metric equipment amounts of 10−8to 10−9M “DNA‐phosphorus” are detectable.Comparable fluorescence yields are produced by DNA‐preparations of different origin and G‐C‐content.Interferences are caused by aldehydes of the type R‐CH2‐CHO, but not by RNA in manifold excess or other ubiquitous substances.Microorganisms can easily be subjected to that procedure. Thus, investigations on growing yeast populations (exhibiting artificial changes from repressed to derepressed oxidative metabolism) clearly demonstrated the anticipated correlations among specific growth rate, cell number, dry mass, protein and DNA‐content. Similar correlations exist among degree of ploidy and DNA‐content of the cell.It was found that the quotient of DNA‐content/cell to dry mass/cell is