Titers of antibodies against hepatitis B surface antigen (HBsAg, anti-HBs, total IgG) in Aotus monkeys immunized at 0 and 8 weeks with either a DNA or protein (HBsAg, 10 µg) vaccine, then with HBsAg at 16 weeks. The DNA vaccine encoded HBsAg under the control of a cytomegalovirus promoter (pCMVA-S) and each monkey received 400 µg at each immunization. The DNA vaccines were given by (1) intramuscular injection of lipid-(CellFECTIN)-associated DNA using the Biojector needleless jet injection system [IM/lipid/Biojector], (2) intradermal injection of DNA in saline with a needle and syringe (ID) plus IM in saline with Biojector [IM/Biojector) [ID + IM/Biojector], (3) ID alone, or (4) ID with added E. coli DNA (100 µg) [ID + E. coli]. Sera was taken at 6, 10, and 17 weeks (18 weeks for IM/lipid/Biojector) and assayed for anti-HBs by end-point dilution ELISA assay. Titers are expressed as the highest plasma dilution that resulted in an absorbance value (OD 450) two times greater than that of nonimmune sera from the same animal, with a cut-off value of 0.05. Each bar represents the group mean (n = 4) and the dots indicate the values obtained for individual animals.