In this paper, we characterize the spin-lattice relaxation T 1 , spin-spin relaxation T 2 , and effective relaxation rate MF of magnetic fluids for magnetic resonance imaging using a high-T c superconducting quantum interference device (SQUID) in microtesla magnetic fields. When the magnetic susceptibility of the magnetic fluid was increased, a broadening of proton nuclear magnetic resonance spectra and a growing spin-lattice relaxation T 1 as well as spin-spin relaxation T 2 were observed. The effective relaxation rate MF increased monotonically from 0 to 13 s −1 when the magnetic susceptibility of the magnetic fluids, relative to tap water, was increased from 0 to 0.0015 emu g −1. We demonstrate the magnetic fluid as an image contrast via a high-T c SQUID in microtesla magnetic fields.