Genomic libraries of Plasmodium falciparum were constructed in the pBR322 plasmid. Using the DNA-mediated gene transfer technique, the genomic libraries were introduced into tissue-cultured mouse cells lacking the enzyme adenine phosphoribosyltransferase. Following selection for the adenine phosphoribosyltransferse phenotype, several colonies were isolated. All clones were shown to possess adenine phosphoribosyltransferase activity and pBR322 sequences. In addition, the Km value of adenine phosphoribosyltransferase (for adenine) from a transformant was found to be identical to that from P. falciparum. These results indicate that the adenine phosphoribosyltransferase gene of P. falciparum was successfully cloned and expressed in a mammalian system.