A two-particle distribution function is derived to calculate the effect of the presence of the particles surrounding a test particle. The average drag force exerted on a particle accounting for binary particle-particle interactions is calculated assuming low particle-Reynolds number flow. The introduction of particle-particle interactions results in the prediction of forces that depend on particle concentration and concentration gradients, size gradients and second derivatives of the particle velocity field. Delta and Poisson size distributions were studied explicitly and some predictions compared with available experimental data, showing good agreement.