The effect of molecular weight, concentration, and structure on 1/T1rho, the rotating frame relaxation rate, was investigated for several proteins using the on-resonance spin-lock technique, for locking fields B1 < 200 microT. The measured values of 1/T1rho were fitted to a simple theoretical model to obtain the dispersion curves 1/T1rho(omega1) and the relaxation rate at zero B1 field, 1/T1rho(0). 1/T1rho was highly sensitive to the molecular weight, concentration, and structure of the protein. The amount of intra- and intermolecular hydrogen and disulfide bonds especially contributed to 1/T1rho. In all samples, 1/T1rho(0) was equal to 1/T2 measured at the main magnetic field Bo = 0.1 T, but at higher locking fields the dispersion curves monotonically decreased. The results of this work indicate that a model considering the effective correlation time of molecular motions as the main determinant for T1rho relaxation in protein solutions is not valid at very low B1 fields. The underlying mechanism for the relaxation rate 1/T1rho at B1 fields below 200 microT is discussed.