Transient diffusion-controlled reactions in a binary suspension of equal-sized spheres are studied on the basis of the time-correlation function of concentration fluctuations in thermal equilibrium. The transient effects are characterized by a memory function, which is calculated in an Enskog type approximation from the nonequilibrium distribution function of a pair of diffusing and converting spheres. In Ising model language, the explicit calculation is performed for hard spheres with a square well Ising interaction and with a magnetic field present. The memory function exhibits appreciable dependence on the parameters of the model.