To construct full-genome spotted microarrays, a large number of PCR primers that amplify the required DNA need to be synthesized. We describe an algorithmic technique that allows one to use fewer primers to achieve this goal. This can reduce the expense of constructing full-genome spotted microarrays considerably. PCR primers are usually designed, so that each primer occurs uniquely in the genome. This condition is unnecessarily strong for selective amplification, because only the primer pair associated with each amplification needs be unique. We also describe the interface to our software, MultiPrimer, that computes a small set of primers for amplification of a given gene set.