We have examined the effects of ingestion of chemically denatured ovalbumin (OVA) in mice. Both 8 M urea-denatured OVA (UD-OVA) and carboxymethylated UD-OVA (CM-OVA) were purified by gel filtration. Specific IgG antibody and systemic delayed-type hypersensitivity (DTH) responses to OVA were not suppressed by CM-OVA fed prior to or after immunization with OVA in complete Freund's adjuvant (CFA). When CM-OVA was used instead of OVA for immunization, serum IgG and DTH responses to CM-OVA were orally tolerized by OVA, but not by UD-OVA or CM-OVA. Studies of antigen uptake in mice using sandwich ELISA tests showed that OVA, but not CM-OVA, was absorbed after antigen ingestion. In vitro studies further demonstrated that CM-OVA was digested much more rapidly than OVA. Moreover, studies using bovine serum albumin (BSA) demonstrated that both IgG and DTH responses to BSA were orally tolerant to BSA, but not to denatured BSA. Finally, studies using human gamma-globulin (HGG), a well-known tolerogen, also found that the IgG antibody response to HGG was not orally tolerized by denatured HGG. These results suggest that complete denaturation of globular proteins may affect their processing and absorption in the gut and thus abrogates oral tolerance induction.