A retroviral vector containing the wild–type p53 gene under control of a β–actin promoter was produced to mediate transfer of wild–type p53 into human non–small cell lung cancers by direct injection. Nine patients whose conventional treatments failed were entered into the study. No clinically significant vector–related toxic effects were noted up to five months after treatment. In situ hybridization and DNA polymerase chain reaction showed vector–p53 sequences in posttreatment biopsies. Apoptosis (programmed cell death) was more frequent in posttreatment biopsies than in pretreatment biopsies. Tumor regression was noted in three patients, and tumor growth stabilized in three other patients.