The humoral immune response to Chlamydia trachomatis 10-kDa heat shock protein (Chsp10) in populations of Russian and French origin was studied by using a recombinant Chsp10 enzyme-linked immunosorbent assay. A physiological but not a serological correlation of Chsp10 exposure with Chsp60 exposure was observed in the Russian population. In the French population studied, there was a significant association between detection of anti-r-Chsp10 immunoglobulin G (IgG) antibodies and chronic genital tract infections. Chsp10 residues 50 to 67 were found to contain an immunodominant although not universal B epitope. Cross-reactions with Chlamydia pneumoniae or Escherichia coli GroES protein are limited but may occur. Our study suggests that detection of anti-Chsp10 IgG antibodies is associated with chronicity of C. trachomatis genital tract infection and does not parallel that of anti-Chsp60 IgG antibodies.