Selected blood proteins were studied during the fifth instar of Locusta by means of acrylamide 'disc electrophoresis' after injection of 3H-labeled blood proteins or 3H-leucine. Label was found to accumulate in the "common protein" fraction in the middle of the instar after injection of labeled blood protein. The specific activity of the other blood proteins studied followed the pattern predicted by their concentration changes. The specific activity of blood proteins after injection of 3H-leucine was generally high during the early part of the instar and fell after 152 h. Most activity was found in the "common protein." A program of protein metabolism for the fifth-instar locust is suggested on the basis of these and earlier results.