A human scFv, 15-9, was selected from a phage display library for binding to murine laminin-1. A diabody was made from the scFv by shortening the linker from 15 to 5 amino acids between the VH and VL sequence. Radioiodinated scFv and diabody were analyzed for size, binding to laminin, and biodistribution in tumor bearing mice. Diabody preparations at concentrations greater than 10 nM were largely dimer forms (similar to60 kDa) as judged by gel filtration, but diluted diabody was eluted as a monomer (similar to30 kDa). At low concentrations the radiolabeled diabody did not bind well to laminin. The 1251 diabody had significantly lower accumulation in tumors than did the scFv when injected at lower concentrations. These data indicate that the diabody dimer dissociates at concentrations of about 10 nM resulting in monomers with no binding activity for laminin and poor tumor homing properties. (C) 2004 Elsevier Inc. All rights reserved.