Six different hybridomas secreting anti-SCM mAb and one control mAb were placed into adult mice along with [14C] amino acids for biosynthetically labeling. After sacrifice, the14C mAb ascites along with serum, heart, kidney, lung and skeletal muscle were recovered. Tissue associated specific radio-activity (SpAc) and microscopic structural analyses were performed. Confirmation of mAb specificity was by both immunodot blots as well as Western blot analysis. Peritoneal injection of measured doses of anti-SCM mAb yielded tissue SpAc confirming theirin vitrospecificities10. Two of the mAb showed strong reactivity to both renal (GBM) and lung basement membrane (LBM), two were mainly GBM reactive and two showed polyreactivity with a marked reactivity to a Z-line antigen. Autoradiographic light microscopy confirmed that the anti-SCM mAbs bound to both GBM and LBM and to Z-line antigen. Titrated doses of the mAb yielded autoradiographic confirmation in which the grain number on the GBM and LBM increased with increasing dose of mAb for all mAb except the control. This effect was not seen in the muscle tissues but anatomical localization at the Z-line was consistent. The major significance of these studies is the demonstration that circulating antibodies to SCM can reactin vivowith normal mammalian antigens adding confirmation to thein vitrospecificity of these cross-reactive anti-SCM mAbs.