Through the development of a panel of murine hybridomas reactive to murine hemoglobin, we have been able to study the processing and presentation of self antigens by antigen-presenting cells. Our results demonstrate that peritoneal macrophages in vivo can process and potentially present the self-antigen hemoglobin. We have extended this finding to show that, directly after removal from the mouse, antigen-presenting cells from a variety of tissues stimulate our hemoglobin-specific hybridomas without any manipulation or addition of exogenous antigen. This constitutes direct functional proof that in a nondisease state self proteins are processed constitutively and can be presented in a fashion similar to that in which foreign antigens are presented. Our demonstration that antigen-presenting cells can process and potentially present self as well as foreign molecules implies that self-tolerance occurs at the level of the T cell. This constitutive processing and presentation of self antigens has potentially far-reaching implications in self-tolerance, autoimmunity, and alloreactivity.