We have shown previously that the i.v. inoculation of allogeneic lymph node cells in rabbits induces the appearance in the serum of an alpha M-serine proteinase complex which behaves in an Ig-turnover assay as any polyclonal B-cell activator (PBA), and that this PBA activity is due to the enzyme. Here, we show that the allogeneic stimulation also induces the appearance in the low molecular weight fraction of the serum (1000-110,000 MW) of an inhibitor which blocks the PBA activity of the complex without affecting the PBA activity of LPS or dextran sulphate. The inhibitor blocked the ability of the enzyme associated with alpha M to degrade Chromozym TRY, a low MW trypsin substrate. The inhibitor also blocked the enzymatic activity of trypsin for large as well as for low MW substrates. Thus, allogeneic stimulation in vivo results in the production, not only of an alpha M-proteinase complex, but also of an inhibitor for this proteinase as well as for trypsin. The appearance of the inhibitor, along with the alpha M-serine proteinase complex as a result of allogeneic stimulation in rabbits, is of interest since a similar alpha M-serine proteinase complex and inhibitor may appear in the serum of patients with rheumatoid arthritis.