abstract: I will describe a result obtained together with Stephan LUCKHAUS and Errico PRESUTTI. We consider a model with two species of particles, representing malignant and normal cells. The basic motions of the malignant particles are independent random walks, scaled diffusively. The normal cells move on a slower time scale and obey an exclusion rule among themselves and with the malignant particles. The competition between the two species is ruled by a coupled birth and death process. We prove convergence in the hydrodynamic limit to a system of two reaction diffusion equations with measure valued initial data.