In the small-gap semiconductor Pb1-x-ySnyMn xTe the magnetic phase transition from a ferromagnetic to a spin-glass state at high carrier concentrations is studied. The authors present new data on magnetization, AC susceptibility and specific heat for various carrier concentrations showing the transition. They also present frequency-dependent AC susceptibility measurements. For the spin-glass sample there is a strong dependence on the measuring frequency, which follows the Vogel-Fulcher law. Furthermore, they calculate the phase boundary in the x-p magnetic phase diagram using the mean random field model. There is a satisfactory agreement between the experimental data and the calculations.