1. The virulence of pathogens and metastatic capacity of cancer cells seems to correlate with the ability to adhere to cells and/or to basement membrane components. A key feature of this mechanism is the expression of specific receptors for the basement membrane protein laminin. Three different receptors have been already described in cells phylogenetically very distant, such as human white blood cells, Trichomonas vaginalis and Staphylococcus aureus, all recognizing laminin with the same range of affinity. 2. We have shown that laminin, which is also found in the circulation, enhances phagocytosis of S. aureus by macrophages in a species-specific fashion. Also, monoclonal antibodies (MAb) raised against the bacterial receptor inhibit the phagocytic enhancement mediated by laminin and recognize laminin-binding proteins in unicellular parasites and mammalian cells. The same Mab 1.H12 elutes a 52-kDa protein from bacterial extracts and a 67-kDa band from cancer cell extracts. Since the MAb is a monospecific reagent, results with 1.H12 strongly suggest an evolutionary conservation of the binding site of phylogenetically different laminin receptors.