Serum albumin has one reactive sulfhydryl (Cys-34) that is one of the important binding sites. Cys-34 is located in the crevice on the surface of the albumin molecule and is therefore restricted in its motion. Bovine serum albumin (BSA) Fraction V forms a transparent gel at pD 4.0 (F-form) in D2O at protein concentrations above 7% (BSA*-gel). We studied the molecular motion of Cys-34 on BSA in the solution and gel states by the vector electron paramagnetic resonance (EPR) method using a maleimide spin label. The rotational correlation times of the spin label bound to Cys-34 in the BSA solution and BSA*-gel were in the order of 10(-6) and 10(-5) s, respectively. A longer rotational correlation time of the Cys-34 spin label in the BSA*-gel suggested that the gel network formed in BSA may drastically slow the motion of Cys-34. The integrated value obtained from the vector EPR spectra also showed an extremely dramatic slowing of the Cys-34 spin label during the gel formation. On the other hand, the values for order parameter and the inclination of the principal axis (z) of the Cys-34 spin label to the rotational axis (mu) were the same in the BSA solution and BSA*-gel.