We demonstrate a new method for performing protein assays with very small volumes ( approximately 1.7 muL to 14 pL). Using a laser to modify local surface energy, we manipulate, fuse, and mix droplets containing horseradish peroxidase and its substrates. A detection limit of approximately 30 attomoles of reacting enzyme was measured by optical absorption. We discuss the possibility of extending this lower limit to zeptomoles of enzyme.