A method has been developed for predicting the effect of an air/ground interface on the fast-neutron flux or dose at large distances from a point isotropic source of neutrons in air. The method yields numerical values for functions f(HS) and f(HD) that may be used to express the fast-neutron intensity as a function of source height HS, receiver height HD, and source-receiver separation distance R, in terms of the corresponding infinite air intensity I(R). Thus I(HS,HD,R) = f(HS)f(HD)I(R). The method is called the “First-Last Collision Model” because it is based on the influence of the ground upon the distribution of “first” collisions of neutrons about the source and of “last” collisions about the receiver. Generalized numerical results have been computed, and means have been developed for applying these results to specific cases* Comparisons of these results with those derived from Monte Carlo calculations, and from experiments performed at the ORNL Tower Shielding Facility and the Nevada Test Site indicate that the first-last collision model predicts the fraction of the infinite air intensity within 5 per cent in almost all cases.