SLOP-I, a thermal multi-group program written for the IBM-704, is discussed. It was used extensively to test the utility of various H/sub 2/O scattering kernels, and to assess the accuracy of other computing methods. The problems solved by the code, the physical approximations in the program and library, the method of solution, and suggested modes of operation are described. Derivaticns of all basic equations and complete input and running instructions are included. (M.C.G.)