The spent activated halloysite and roasted bentonite (after 10 days of air filtration) were composted for 100 days with poultry litter, straw and peat. The composts were analyzed for chem. compn. The toxicity of the composts was evaluated by the Phytotoxkit test (with Lepidium sativum, Sinapis alba and Sorghum saccharatum) and by OECD earthworm acute toxicity test (with Eisenia fetida). Moreover, the effect of composts on Sinapis alba biomass and chem. compn. of plants was assessed in the pot tests. The activated halloysite compost was not applicable as a soil amendment due to its toxicity to the earthworms and plants. It contributed also to a decrease in mustard yield. The addn. of the composts resulted in an increase in nitrate N, Cu, Zn and Cr contents in mustard seeds. The bentonite compost was toxic neither to earthworms nor to plants.