We report a study of the magnetic properties of transition-metal doped Zn1−xTMxO (TM=Mn, Co, Fe). Polycrystalline powder samples were synthesized by both solid-state and liquid-phase reactions. From the Curie–Weiss behavior of susceptibility at high temperatures, it was found that the TM–TM interaction is dominated by antiferromagnetic coupling with effective nearest-neighbor exchange constants J=−90 to −30 K. The magnetization data measured at low temperature as a function field H are fit to a parameterized Brillouin function to obtain the effective concentration xeff of magnetically active TM2+ ions. As x increases, the fraction of magnetically active ions, xeff/x, decreases. This is ascribed to an increase in average AF interaction between doped magnetic spins as the average distance between them decreases with an increase in x.