A FORTRAN program to analyze homology of letter strings (nucleotide or amino acid sequences) and to display the result in the form of a dot matrix is presented. The program is generally usable, user-friendly and has a number of options (filtering, "fudging," i.e., consideration of groups of homologous residues, and screening, i.e., display of only particular groups of residues) which greatly potentiate its analytical power.