A characteristic feature of infections with the nematode parasite of mice Heligmosomoides polygyrus, is a marked IgG1 hypergammaglobulinaemia. A possible source for this immunoglobulin has recently been demonstrated, through evidence that H. polygyrus adult worm homogenate (AWH) can induce the in vitro production of non-specific IgG1 from mouse lymphocytes. To determine the interactions between this immunoglobulin and the parasite, the ability of IgG1 to bind to AWH of H. polygyrus was investigated. Protein (Western) blotting indicated that mouse monoclonal antibodies are able to bind non-specifically to selected parasite antigens. Furthermore, by binding H. polygyrus adult worm homogenate to cyanogen bromide (CNBr)-activated Sepharose CL-4B, an affinity column was prepared which could be used to efficiently purify mouse IgG1 monoclonal antibodies. These antibodies were eluted from the affinity column and still retained their original specificity. These results indicate that H. polygyrus not only induces the production of non-specific IgG1 by the host, it can also bind this immunoglobulin to its own specific proteins. Thus, it is possible that IgG1 produced during a primary infection with H. polygyrus may not entirely benefit the host.