Four samples of cummingtonite-grunerite series in various crystallization habits were tested in vitro. The cytotoxicity to Chinese hamster ovary cells and hemolysis to sheep erythrocytes were inversely proportional to the structural faults and surface defects of the minerals. At a comparable surface area, asbestiform grunerite (UICC amosite), semi-asbestiform cummingtonite, acicular cummingtonite, and acicular grunerite were found to be cytotoxic and hemolytic in a decreasing order. The influence of particle size on hemolysis and cytotoxicity was observed with acicular grunerite. Although samples of relatively large particle size were found to be inert, samples of smaller particle size were cytotoxic as well as hemolytic. No apparent relationship between surface charge and hemolysis as well as cytotoxicity was observed.