Extracellular metallothionein (Zn,Cd-MT) has previously been shown to be a potent inducer of lymphocyte proliferation and to synergize with polyclonal activators in proliferation assays. In this report, the effects of metallothionein on the development of humoral responsiveness are examined. In vivo, the specific anti-ovalbumin (OVA) IgG response was diminished by co-injection of Zn, Cd-MT, while total IgG levels remained unchanged. A similar reduction was also observed when Zn,Cd-MT was administered during the development of an anti-sheep red blood cell (sRBC) humoral response. When amounts of Zn and Cd equimolar to that associated with the Zn, Cd-MT were co-injected with OVA, humoral responsiveness was enhanced, in contrast to the suppression seen with Zn, Cd-MT. Apothionein lacking the available thiols associated with native Zn, Cd-MT had no effect on the development of humoral immunity. These results point to the thiols associated with the protein as the important determinants in the observed immunosuppression and this is supported by the capacity of UC1MT, a new monoclonal anti-MT antibody, to reverse MT mediated immunosuppression. No evidence was found to suggest that Zn,Cd-MT was interacting directly with OVA. Finally, in vitro experiments with LPS-stimulated splenocyte production of IgM correlated with the in vivo observations of Zn,Cd-MT. These data provide evidence for a significant role for MT in the development of metal-mediated immunomodulation and suggest that MT may also possess immunomodulatory functions under circumstances where MT is synthesized in the absence of heavy metal stress. Furthermore, it may be possible to take advantage of this system to exogenously manipulate the development of the immune response.