Mass rearing screwworm for the eradication program is one of the largest insect rearing projects in the world. The production facility, located in Mexico, currently produces about 100 million flies per week. The larval diet used for mass rearing consists of spray-dried blood, spray-dried egg, and a milk substitute, mixed with a gel and water. Nearly 15,000 kg of ingredients at a cost of about $25, 000 per week is required for this purpose. Recent experiments indicate that the gelling agent, the most expensive ingredient, can be replaced by cellulose fiber, resulting in a significant reduction in the cost. To further economize the mass production, tests were conducted to determine if fresh bovine blood, available free from local slaughter houses, can replace the expensive imported spray-dried blood. The diet was prepared by mixing 16 ml of fresh blood, mechanically defibrinated, with other ingredients (4 g egg, 4 g milk, 1.8 g gel or 5 g cellulose fiber) and adding water to make 100 ml of the diet. For 100 mg of eggs (~ 2000 eggs) per rearing tray, a total of 1.5 liters of diet was used. Control tests were run using the standard gelled diet with spray-dried blood plus the other ingredients. Results show that the larval, and pupal weight, and the number of pupae per tray were not significantly different in test and control experiments, indicating that the fresh blood supported the larval growth as good as the spray-dried blood.