To investigate the cytotoxicity of three high copper amalgams (HA 1, HA 2 and HA 3) which are currently available in Japan, and compare with that of the conventional amalgams, i. e., fine cut, spherical and copper amalgams, a cell culture system with L strain fibroblasts was used. Results were assessed by cell multiplication rate calculated from cell nuclei counting and cell morphology. The results obtained were as follows: 1. Amalgam alloys of two of the high copper amalgams tested (HA 3 and HA 1) as well as two of the conventional amalgams, i. e., fine cut and spherical amalgams did not give adverse effects to the cells. However, the alloys of the other high copper amalgam, HA 2, and copper amalgam yielded intense cytotoxicity. 2. As to the triturated amalgams, two of the high copper amalgams (HA 3 and HA 1) showed a very similar effects to the cell to the fine cut and spherical amalgams. At three hours after trituration, depression of cell growth diminished in two of the high copper amalgams(HA 3 and HA 1). However, the other high copper amalgam, HA 2, yielded intense and persistent cytotoxicity even at 24 hours after after trituration. 3. Cell morphology of the experimental groups confirmed the above result of the rate of cell multiplication. 4. A similar biological effect to the results in several previous reports with three conventional amalgams in vitro was confirmed. It was discussed that difference of biological effects in vitro among the three high copper amalgams might be attributed to the alloy composition. It is considered that the present results will be able to lend suggestions to the use of the high copper amalgams.