Tetra-primer ARMS-PCR is used extensively as a low cost, single PCR assay requiring no post-PCR manipulation. The design of successful primers depends on a number of variables such as melting temperatures, GC content, complementarity and selection of mismatch bases. The optimal selection of primers can be achieved in an automated way using a program which evaluates candidate primers for a given sequence. The Primer1 software was developed originally for use in the context of restriction fragment length polymorphism analysis using gel electrophoresis. However, recent applications have been more diverse, reviewed here, and we present an overview of the Primer1 software for primer design and web-service. We have updated the Primer1 program, and provide more complete details of the implementation. We also provide test data and output. The program is now available on a new, efficient, LAMP web service for users at: http://primer1.soton.ac.uk/primer1.html.