We have identified five normal individuals without known exposure to mouse immunoglobulin with “preexisting” human anti-mouse antibody (HAMA) against a panel of four mouse monoclonal antibodies and polyclonal mouse IgG. Competitive inhibition by polyclonal mouse IgG of HAMA binding to monoclonal antibody coated beads demonstrates the mouse constant region specificity of these sera. Lack of such inhibition by polyclonal human IgG eliminates polyclonal rheumatoid factors as an explanation. Lymphoblastic transformation studies in these five persons failed to demonstrate a memory T-cell response to the four mouse monoclonals or polyclonal mouse IgG. Positive controls included T-cell responses to streptolysin O in these five individuals as well as responses to monoclonal antibody D612 in three patients following treatment with that antibody. This lack of T-cell immunity to mouse IgG suggests that “preexisting” HAMA is the product of inadvertent cross-reactivity with murine constant region by antibodies directed against other antigens. Therefore, “preexisting” HAMA should pose no risk of anamnestic or allergic response in patients considered for murine monoclonal therapy. © 1992, American Association for Cancer Research. All rights reserved.