One anticipated advantage of DNA immunization is the potential to create multivalent vaccines. We have examined the effects of mixing plasmids into single formulations using plasmids expressing four different membrane bound glycoproteins from bovine herpesvirus-1 (BHV-1), bovine parainfluenzavirus-3 (bPI3) and human influenza virus (HINF). Plasmids were delivered by intradermal injection into the tails of mice and the types of responses generated were clearly affected by the expressed antigen. Plasmids expressing glycoproteins B and D of BHV-1, and the haemagglutinin/ neuraminidase of bPI3, generated responses with a predominance of IgG1, suggestive of a Th2 type of response. In contrast, the plasmid expressing HINF haemagglutinin induced an antibody response biased towards IgG2a, indicating a Th1 type of response. In most instances the mixing of plasmids had only slight effects on the magnitude or bias of the responses to the individual components. However, under certain conditions we found that addition of a second plasmid converted an IgG2a biased response to a response with primarily IgG1 antibody. The reverse situation (i.e. an IgG1 conversion to IgG2a), however, was not found. These findings have important implications for the development of multivalent vaccines.