Neutrophil granulocytes migrate rapidly into three-dimensional gels of native collagen fibres, the leading front of the cell population moving approximately 100 micron/hour when the cells are unstimulated. Migration occurs in the apparent absence of adhesion to collagen fibres, as neutrophils do not adhere to collagen-coated glass and are unable to locomote over collagen-coated surfaces, probably due to lack of traction with the substratum. It would appear, therefore, that the mechanism of neutrophil movement through three-dimensional collagen gels is fundamentally different to the mechanism of migration over a planar substratum. Freshly isolated blood monocytes do not invade collagen gels. However, if monocytes are cultured on serum-coated plastic, approximately 80% of the cells spontaneously detach from the culture surface over a 48 hour period: these detached cells invade collagen gels, although at a much slower rate than neutrophils.