Long-term expression of a reporter gene has previously been reported in skeletal and cardiac muscles after direct injection of naked plasmid DNA. In this study, we have shown that the direct injection of free plasmid DNA into mouse melanoma BL6 solid tumor can also result in a high level of transfection. THe average amount of chloramphenicol acetyltransferase (CAT) expressed by injecting 30 micrograms plasmid DNA containing a CAT gene into a single BL6 tumor was 1.9 +/- 1.0 ng, which is comparable to that reported in the skeletal muscle. Cationic liposomes, Lipofectamine and DC-chol/DOPE, inhibited gene expression in a dose-dependent manner. Transgene expression by free DNA persisted for at least 10 days. The size of tumor did not seem to affect the gene expression, but proper choice of a diluent solution for DNA was an important factor. Genes driven by the CMV promoter were expressed much more efficiently than genes driven by the SV40 or T7 promoter. Optimal dosage of injected DNA was from 30 to 70 micrograms per tumor. Other mouse melanomas, human melanomas and cervical carcinomas are also able to express directly injected plasmid DNA, but the transfection efficiency is lower than the BL6 tumor. Direct injection of free plasmid DNA is a simple and effective approach and might be a potential method for cancer gene therapy.