Plasmacytomas were readily induced in NZB mice by three i.p. inoculations with pristane (2, 6, 10, 14-tetramethylpentadecane). In comparison with comparable induction regimens in BALB/c mice a) the latent period for plasmacytoma development was significantly longer in NZB's; b) the frequency of IgA paraproteins produced by NZB tumors was much reduced and that of IgG (IgG1, IgG2a, IgG2b, IgG3) was significantly increased; and c) the frequency of carbohydrate-binding paraproteins was 10-fold lower. There was a high frequency of primary ascites containing more than one paraprotein, but none of 25 such tumors produced more than one immunoglobulin class on serial transplantation.