EC50 values of different nitroparaffins were determined by a photobacterium phosphoreum toxicity test. Octanol/water partition coefficients were determined by a reversed phase HPLC method. From these experimental data quantitative structure activity relationships were deduced which point to a dependence of toxicity on a 'form factor' that includes the number of methyl and methylene groups in a molecule. The hypothesis will be formulated that the specific effects depend on the number of methylgroups and an equation for specific effects is given