Human polymorphonuclear leukocytes mainly protect the body against invading microorganisms. Therefore, cells must migrate to the site of infection, whereby they traverse the endothelial cell layer of the blood vessels, basement membranes and the connective tissue. We investigated the unknown mechanism of penetration of basement membranes by scanning electron microscopy. Our findings indicate a three-step mechanism--adhesion, locally limited proteolytic degradation of matrix components and mechanical loosening of the matrix. Finally, cells locomote in the stroma tissue to the site of infection. These processes are induced by formyl-peptides amongst others. In addition, an increased release of collagenase induced by formyl-peptides was observed. In contrast, incubation with prostaglandin F2 alpha led to significantly higher levels of secreted collagenase, while elastase failed to be detected in the supernatant of stimulated cells. Since it has been shown that collagenase is secreted in a latent form, we have focussed our attention on the activation mechanism of the proenzyme, which was investigated by determination of the N-terminal sequences of intermediate and final, activated forms.