Human neutrophils generally function adherent to an extracellular matrix. We have previously reported that upon adhesion to laminin- or fibronectin-coated, but not uncoated, plastic there is a depolymerization of actin in neutrophils. This phenomenon was not affected by inhibitors of the more well-studied components of the signal transduction pathway, specifically, pertussis toxin, an inhibitor of G-proteins, H-7 or staurosporine, inhibitors of protein kinase C, or herbimycin A, an inhibitor of nonreceptor tyrosine kinase. We therefore focused our attention on actin-binding proteins and measured the changes in the partitioning of gelsolin between the Triton X-100-soluble and -insoluble cellular fractions which occur upon neutrophil adhesion by means of quantitating anti-gelsolin antibody binding to aliquots of these fractions. It was found that approximately 90% of the total cellular gelsolin was found in the Triton X-100-soluble fraction in suspended cells, but that upon adherence to either fibronectin- or laminin-coated plastic about 40% of the soluble gelsolin could be detected in the insoluble fraction. This effect was not observed in cells adherent to uncoated plastic, wherein more than 90% of the gelsolin was found in the soluble fraction. Results of immunofluorescence microscopy of these cell preparations was consistent with this data. A gelsolin translocation to the insoluble cellular actin network may account for a part of the observed actin depolymerization.