The different effects of Ce(3+), Cd(2+), and Hg(2+) on the activities and secondary structure of trypsin were studied. The results showed that trypsin activity was increased substantially by Ce(3+) in 0.5-5 micromol/L concentration, but the activity was decreased significantly by Cd(2+) or Hg(2+) in 0.5-5 micromol/L concentration. The ultraviolet-visible spectrum of trypsin with 4 micromol/L Ce3+ treatment was the same as that of the control, but the 232-nm characteristic peak of trypsin with 4 micromol/L Cd(2+) or Hg(2+) treatment was blue-shifted and the peak intensity weakened. The circular dichroism (CD) spectrum of trypsin with 4 micromol/L Ce(3+) treatment was similar to that of the control. The secondary structure of trypsin did not change with Ce(3+) treatment. However, the CD spectrum of trypsin with 4 micromol/L Cd(2+) or Hg(2+) treatment was different from that of the control and Ce(3+) treatment. The secondary structure of trypsin with Cd(2+) or Hg(2+) treatment changed greatly; for example, the alpha-helix and beta-sheet contents were reduced significantly, the beta-turn was enhanced greatly, and the random coil contents increased or decreased.