We investigated the effect of three monoclonal anti-idiotype antibodies (anti-M104E) on various functions of MOPC 104E myeloma cells in vitro. The antibodies used were N-20-2 [im–munoglobulin M (IgM), BALB/c], SJL18-1 [IgM, BALB/c X SJL Fl], and CD3-2 [immunoglobulin G1 (lgG1), BALB/c x A/J F1]. The two IgM antibodies were very efficient in blocking surface M104E IgM as shown by rosette inhibition, whereas the lgG1 isotype was not very effective. The reexpression of surface M104E IgM was different from antibody to antibody. The secretion of M104E IgM by MOPC 104E cells was partially blocked by the two IgM antibodies, but the lgG1 antibody had no effect. All three anti-idiotype antibodies inhibited the stem cell renewal activity of MOPC 104E cells assayed by colony formation assay. On the other hand, in suspension culture, the two IgM antibodies inhibited the growth of MOPC 104E cells in the absence of complement or effector cells of antibody–dependent cellular cytotoxicity, but lgG1 antibody had no effect. The starting tumor inoculum size was critical in the observations of the effects seen on both the growth and the colony–forming activity of MOPC 104E cells. The results of this study show the functional differences between various monoclonal anti-idiotype antibodies and also indicate that some anti-idiotype antibodies can inhibit the growth of MOPC 104E myeloma cells directly without any help of complement or effector cells of antibody–dependent cellular cytotoxicity. copyright. © 1986, American Association for Cancer Research. All rights reserved.