Tumor cell adhesion and migration to laminin are important events during invasion and metastatic spread. Galectin-3, a multifunctional member of the galectin family, binds specifically the poly-N-acetyllactosamine residues of laminin and has been implicated in tumor invasion and metastasis. Galectin-3 is multimerized by transglutaminase, an enzyme that catalyzes cross-linking between glutamine and other aminoacid residues. In this study, we examined the consequences of transglutaminase-mediated galectin-3 oligomerization on the interactions between cancer cells and laminin. We first demonstrated that human galectin-3 is cross-linked by guinea pig liver transglutaminase, forms oligomers, and incorporates the marker 5-(biotinamido) pentylamine. Expression of transglutaminase activity in the A375 and A2058 human melanoma cell extracts was revealed by its ability to induce galectin-3 oligomerization and 5-(biotinamido) pentylamine incorporation. Transglutaminase-treated galectin-3 did not affect adhesion or migration of the melanoma cells to laminin but consistently induced a significant increase of the percentage of cell spreading compared to the control (23.5 +/- 2.3%, vs. 10.6 +/- 1.9% at 180 min, p < 0.05), or to untreated galectin-3 or transglutaminase alone. Our study is the first demonstration that human galectin-3 is oligomerized by transglutaminase with, as a consequence, a specific effect of melanoma cell spreading on laminin. This phenomenon could be of significance in the modulation of cancer cell interactions with laminin during tumor invasion and metastasis.