Denatured type-I collagen was found to bind to Chlamydia trachomatis elementary bodies (EBs) in a time-dependent and specific manner. Specificity was tested by having a large excess of other proteins present in the binding mixture. Only denatured type-I collagen efficiently competed for binding. Radiolabelled fibronectin did not bind under the test conditions used. The binding was temperature-dependent and the interaction increased at the melting temperature of the collagen. Evidence was found for two binding sites: one with high affinity (Kd 3.3 x 10(-9)) and one with low affinity (Kd 1.7 x 10(-7)), with an estimated number of binding sites per EB of 590 and 2900, respectively. The interaction between C. trachomatis and collagen may also be relevant in vivo, since 50% binding occurred at 37 degrees C. The binding to denatured collagen may be of importance for the development of sexually acquired reactive arthritis.