Binding of receptor-recognized forms of tetrameric human alpha 2-macroglobulin (alpha 2M*) to a macrophage signaling receptor induces cAMP synthesis, increases in inositol 1,4,5-triphosphate (IP3) synthesis, and a concomitant rise in cytosolic free calcium ([Ca2+]i). The alpha 2M* signaling receptor is coupled to a pertussis-toxin insensitive G protein. Binding of alpha 2M* also occurs to the low density lipoprotein receptor-related protein/alpha 2M receptor (LRP/alpha 2MR), but this binding does not induce signal transduction. Rat alpha 1-inhibitor-3 (alpha 1I3) is a monomeric member of the alpha-macroglobulin/complement superfamily. Like alpha 2M, it can react with proteinases or methylamine which induces a conformational change causing activated alpha 1I3 to bind to LRP/alpha 2MR. We now report that alpha 1I3-methylamine binds to the macrophage alpha 2M* signaling receptor inducing a rapid rise in the synthesis of IP3 with a subsequent 1.5- to 3-fold rise in [Ca2+]i. alpha 1I3-methylamine binding to macrophages also caused a statistically significant elevation in cAMP. Native alpha 1I3, like alpha 2M, was unable to induce signal transduction. alpha 1I3 forms a complex with alpha 1-microglobulin, which has a distinct conformation from alpha 1I3 and is recognized by LRP/alpha 2MR. This complex also induces an increase in [Ca2+]i comparable to the effect of alpha 1I3-methylamine on macrophages. It is concluded that activation of alpha 1I3 by methylamine or binding of alpha 1-microglobulin causes similar conformational changes in the inhibitor, exposing the receptor recognition site for the alpha 2M* signaling receptor, as well as for LRP/alpha 2MR.