A method of sample-size determination for use in attempts to replicate experiments is described. It is appropriate in situations where there is uncertainty about the magnitude of the effect under investigation. The procedure uses information supplied by the original experiment to establish a distribution of probable effect sizes. The sample size to be used in a replication study is that which provides an expected power of the desired amount over the distribution of probable effect sizes A FORTRAN 77 program is presented that permits rapid calculation of sample size in replication attempts employing comparisons of means, correlation coefficients, or proportions.