Human serum albumin (HSA) is known to exist as N (pH similar to 7), B (pH similar to 9), and F (pH similar to 3.5) isomeric forms and an equilibrium intermediate state (1) accumulate in the urea induced unfolding pathway of HSA around 4.8-5.2 M urea concentrations. These states displayed characteristic structure and functions. To elucidate the ciprofloxacin (CFX) binding behavior of HSA, the binding of ciprofloxacin with these conformational states of human serum albumin (HSA) has been investigated by fluorescence spectroscopy. The binding constant (K) for N, B, F, and I conformation of HSA were 6.92 x 10(5), 3.87 x 10(5), 4.06 x 10(5), and 2.7 x 10(5) M-1 and the number of binding sites (n) were 1.26,1.21, 1.16, and 1.19, respectively. The standard free energy changes (Delta G(binding)(0)) of interaction were found to be -33.3 (N isomer), -31.8 (B isomer), -32 (F isomer), and -30.0 W mol(-1) respectively. By using unfolding pathway of HSA, domain 11 of HSA has been assigned to possess binding site of ciprofloxacin. Plausible correlation between stability of CFX-N and CFX-B complexes and drug distribution have been discussed. At plasma concentration of HSA fraction of free CFX, which contributes potential to its rate of transport across cell membrane, was found to be similar to 80% more for B isomers compared to N isomers of HSA. The conformational changes in two physiologically important isomers of HSA (N and B isomers) upon ciprofloxacin binding were evaluated by measuring far, near-UV CD, and fluorescence properties of the CFX-HSA complex.