Hybridomas derived from mice sham-immunized with saline were found to secrete 'natural' antibodies with a wide range of specificities. A high response to bovine serum albumin (BSA) and in particular BSA conjugated with oxazolone was observed routinely. The oxazolone-BSA response was probably directed toward antigenic sites exposed as a result of the coupling procedure; only 2% of the oxazolone-BSA-binding supernatants also bound to oxazolone-ovalbumin. An unexpected cross-reactivity was seen between the oxazolone-BSA-binding supernatants and an 18-amino-acid peptide that forms part of the VP1 capsid protein of poliovirus serotype 3. Some supernatants were also found to react with all proteins tested, including the synthetic poliovirus peptide; this reactivity was maintained following cloning. Primary immunization resulting in the generation of antibodies to the injected antigen nevertheless had no effect on the repertoire of 'natural' antibodies produced. Injection of the T-cell mitogen concanavalin A increased the frequency of 'natural' antibodies. This effect was enhanced when ConA was injected together with antigen.