A general purpose computer program for the calculation of a matrix of tetrachoric correlations is described. This program was developed for use in adaptive (and other) testing research for examining the unidimensionality assumption in latent trait theory, in conjunction with available factor analysis programs. Several other potential applications, as well as details for its use, are described. The program accepts as input raw dichotomous data, reduced joint frequency data, or joint and marginal proportions, for up to 75 items. Output options include the tetrachoric correlation matrix, the matrix of phi coefficients, fourfold frequency tables for every item pair, a joint frequency matrix (which reduces all the information in the fourfold tables to a square matrix with order equal to the number of items), and a pair-by-pair listing of input proportions and output correlations which permits testing the program against published tables of the tetrachoric correlation. Variable input and output formatting makes the program convenient to use in conjunctions with other analyses by packaged statistical programs. Examples of input and output are presented. A complete FORTRAN IV listing is included. (Author)