In order to study the effect of applied magnetic field on the superparamagnetic relaxation behavior of small Co particles, magnetization measurements were carried out on as-prepared and annealed granular samples ofCo20Ag80andCo25Ag75.Values of the superparamagnetic blocking temperatureTB−were obtained from the characteristic peak in the zero-field-cooled magnetization. Consistent with existing models, it was found that the initial decrease ofTB−with applied magnetic field is quadratic. An estimate of the magnetic anisotropy “energy density”Kuyielded a value which is two orders of magnitude greater than the value for bulk cobalt. The results reported here underscore the importance of considering the effect of superparamagnetic relaxation on the performance of nanostructured magnetic materials. ©1998 American Institute of Physics.