1. The estimation of dose-response relationships is an integral and legally necessary part of the routine regulatory process for herbicides. For each herbicide, plants of both target and non-target species are exposed to different levels of the chemical, and effects on mortality and performance expressed as LD50 and ED50 values (respectively, the dose at which 50% of plants die and the dose at which plants show a 50% response to herbicide). Thereafter, LD50 and ED50 values may provide comparative information between herbicides and species. There is little published on the effects of plant density on dose-response relationships. 2. We used the herbicide 2,4-D amine and Agrostemma githago as a model system to investigate the effects of plant density on the dose-response relationship (ED50) between 2,4-D and A. githago biomass measured as fresh weight. 3. Plants were grown in a controlled environment room at both low and high densities (two and 64 plants per pot) for 2 weeks, sprayed with 2,4-D using a precision sprayer, and then harvested after a further 2 weeks. The ED50 values were significantly greater when the plants were grown at high density (616 g active ingredient ha-1) than those grown at low density (42 g active ingredient ha-1); a 15-fold difference. 4. Mathematically, it was shown that a simple multiplicative relationship exists between ED50 and dose received when plants are grown at different densities, all other conditions being equal. 5. To explore the underlying effects of competitive processes arising from different densities, we experimentally investigated three phases where competition could be influencing the response of plants: (i) before spraying, (ii) at the time of spraying and (iii) after spraying. We used a sequential series of experiments to determine which phase was contributing most to the observed difference between ED50 values obtained from plants grown at low and high density. 6. It was shown that the competition between plants after spraying was the most likely phase to have contributed to the observed difference in ED50 values between the two densities. 7. These results demonstrate that trials used in the pesticide regulatory process ought to test not only different doses of pesticide but also different densities of plants (both crop and weed).