To produce an immune reaction against a foreign protein usually requires purification of that protein, which is then injected into an animal. The isolation of enough pure protein is time-consuming and sometimes difficult. Here we report that such a response can also be elicited by introducing the gene encoding a protein directly into the skin of mice. This is achieved using a hand-held form of the biolistic system which can propel DNA-coated gold microprojectiles directly into cells in the living animal. Genetic immunization may be time- and labour-saving in producing antibodies and may offer a unique method for vaccination.