We have produced a monoclonal antibody to human basement membrane Type IV collagen. The antibody reacts with the pepsin-resistant, collagenase-sensitive domain of Type IV collagen isolated from placental membranes, but not with human collagens of Types I, II, III, V, 1alpha, 2alpha, and 3alpha. The antibody precipitates biosynthetically labeled human Type IV procollagen, and the precipitate contains both the alpha1 (IV) and alpha2 (IV) chains, suggesting the occurrence of both of these chains within the same triple-helical molecule. When used in indirect immunofluorescence, the antibody gives brilliant staining of basement membranes from a variety of human tissues but does not stain tissues of bovine, canine, rabbit, rat, or mouse origin. It is suggested that this antibody will be of value in research on the structure of human basement membrane collagen, on the distribution of this collagen in various basement membranes, and particularly for the study of basement membranes in normal human development and pathologic processes.