The fimbria-associated MrkD_1P protein mediates adherence of type 3 fimbriate strains of Klebsiella pneumoniae to collagen type V. Currently, three different MrkD adhesins have been described in Klebsiella species, and each possesses a distinctive binding pattern. Therefore, the binding abilities of mutants possessing defined mutations within the mrkD_1P gene were examined in order to determine whether specific regions of the adhesin molecule were responsible for collagen binding. Both site-directed and chemically induced mutations were constructed within mrkD_1P, and the ability of the gene products to be incorporated into fimbrial appendages or bind to collagen was determined. Binding to type V collagen was not associated solely with one particular region of the MrkD_1P protein, and two classes of nonadhesive mutants were isolated. In one class of mutants, the MrkD adhesin was not assembled into the fimbrial shaft, whereas in the second class of mutants, the adhesin was associated with fimbriae but did not bind to collagen. Both hemagglutinating and collagen-binding activities were associated with the MrkD_1P molecule, since P pili and type 3 fimbriae carrying adhesive MrkD proteins exhibited identical binding properties.