Evidence from circular dichroism studies suggests that the binding of calicheamicin to DNA induces an optically detectable conformational change of B-form DNA. CD titration of a dodecamer containing a TCCT binding/cleavage site with calicheamicin gamma(1)(I) the aromatized calicheamicin epsilon brings about a decrease primarily in the ellipticity of the positive 270 nm signal. Binding constants for calicheamicin gamma(1)(I) and epsilon with this dodecamer were estimated to be 9x10(5) and 5x10(4) M(-1), respectively, based on analyses of the CD changes.