A calculation method developed for mixed-fuel lattices, consisting mainly of natural uranium rods and a small number of enriched rods isolated from each other, is studied with the aid of pulsed -neutron and exponential experiments. The experiments and theory are compared by means of the asymptotic spatial and time decay constants. In the theoretical calculations the natural uranium lattice is homogenized and the multigroup diffusion theory is applied; the enriched rods are described heterogeneously by using the monopole approximation. A separate transport theoretical cell calculation is carried out for the monopole boundary condition to obtain the relationship between the neutron current and flux at the surface of the lattice cell corresponding to an enriched rod. The results show that this kind of treatment is valid, although the cell calculation, where the axial flux dependence is disregarded, causes an error in the exponential experiments that is opposite to and greater than that in the pulsed-neutron experiments.