The algorithms that drive the map+ program for locus-oriented linkage mapping are presented. They depend on the enhanced location database program ldb+ to specify an initial comprehensive map that includes all loci in the summary lod file. Subsequently the map may be edited or order constrained and is automatically improved by estimating the location of each locus conditional on the remainder, beginning with the most discrepant loci. Operating characteristics permit rapid and accurate construction of linkage maps with several hundred loci. The map+ program also performs nondisjunction mapping with tests of nonstandard recombination. We have released map+ on Internet as a source program in the C language together with the location database that now includes the LODSOURCE database. The anonymous ftp is cedar.genetics.soton.ac.uk and the World Wide Web address is http://cedar.genetics.soton.ac.uk/public_html.