In this report we describe the production of recombinant fusion proteins of the HLA-DRw6 beta chain and the murine Ia-associated invariant chain. cDNAs encoding the human HLA-DRw6 beta chain and the murine Ia-associated invariant chain were introduced into bacterial expression plasmids. These plasmids direct the synthesis of the respective molecules as fusion proteins of the bacteriophage MS-2 polymerase by E. coli. Fusion proteins purified from crude E. coli lysates were used to raise antisera in rabbits. These antisera were able to immunoprecipitate biosynthetically labelled class II and invariant chain antigens. Additionally, two anti-DR antisera were raised against single domains of the HLA-DR beta chain thus generating reagents with a defined fine specificity. The anti-murine invariant chain serum was shown to cross-react with the human invariant chain and therefore may be useful for studying invariant chain and Ia antigen expression in different species. The method described here permitted us to produce large quantities of immunologically relevant proteins, for use in the production of polyclonal and monoclonal antibodies. Soluble fragments of the fusion proteins representing certain DR domains may also be useful in functional immunological studies.