Magnetic viscosity measurements were made on native horse-spleen ferritin in zero applied magnetic field at temperatures between 21 and 21 K. The data have been used to calculate the apparent magnetic-moment-weighted energy barrier distribution for the sample of ferritin. The distribution is composed of a log-normal distribution plus a second distribution that is well described by an exponential decay of barrier frequency with increasing barrier height. The two distributions contribute approximately equally to the overall distribution. The log-normal distribution has its peak at an energy barrier of approximately 3 x 10(-21) J, while the decay constant for the second distribution has a value of approximately 2 x 10(-21) J. The existence of the low-energy barrier distribution with exponentially decaying shape in conjunction with the observation of shifted field-cooled magnetic hysteresis loops is interpreted as strong evidence for the existence of multiple interacting magnetic entities within each ferritin particle.