Reactivity of tumor-derived Id proteins with the treatment MoAb 7D11 and with sera from immunized mice. Sera from individual mice were absorbed against polyclonal human Ig and on normal Ig from patient C.J. The absorbed sera were then reacted at a final dilution of 1:50 in 2% BSA/PBS with the 7D11-idiotope–positive proteins (2A12 and 1H1), with mutated 7D11-idiotope–negative Id variants (1B11 and a to h), and with class-matched irrelevant human Ig or with C.J. normal Ig. Bound murine Ig was detected with HRPO-conjugated goat anti-murine IgG Abs (human-absorbed). The mean OD at 405 nm per vaccine group of 5 mice is given for each Id protein. Reactivity with the treatment MoAb 7D11 controlled for the presence of the specific idiotope in Id protein 2A12 and 1H1 and its absence in the Id variants a to h, 1B11, and the human control Igs. Sera from mice immunized with irrelevant Igs were negative in this assay.