A computer routine was written to enable an efficient, yet accurate, interpolation of the basic probabilities required in integral transport calculations of single lattice, as well as multicell, structures. These are The tables within which the routine interpolates contain remainders between accurate probabilities to respective analytical approximations. There are ∼4000 entries for a cylindrical or spherical geometry and 50 for slab geometry. The accuracy is generally within a few tenths of a percent relative error for all the probabilities and can be much lower. The range of optical thicknesses covered is 0 to 20. All the probabilities required for a given layer can be generated on a CRA Y-XMP in a 5 × 10-6 s. A single Dancoff probability can be generated in ∼2.7 × 10-6 s.