Cytotoxicity in vitro and anti-tumour activity in vivo of magnesium alloys Mg-6%Ag and Mg-10%Gd was studied. It was shown that specifically designed and thermomechanically processed Mg alloys produced a sizeable cytotoxic effect on PC-3 line tumour cells in vitro through inhibition of their proliferation. A pronounced grain refinement in combination with the formation of second phases and precipitates attained by means of equal-channel angular pressing (ECAP) accellerated the degradation and gave rise to enhanced anti-tumour activity of both alloys. The gadolinium-containing alloy outperformed the silver-containing one substantially. In an in vivo assay, intratumoural implantation of pins made from both alloys in the homogenised and the ECAP states in mice with inoculated B16 melanoma gave rise to an indubitable anti-tumour effect. It was documented in a decrease of Ki-67(+) cells and the occurrence of regions of destruction of tumoural tissue that were filled with gas evolving in the course of biodegradation of the implants. The data obtained suggest that intratumoural implantation of gadolinium-containing magnesium alloys can be considered for therapy of inoperable or chemoresistant forms of cancer.