Abstract 1. 1. The protein concentration in the hemolymph of fifth instar and adult milkweed bugs was determined under three experimental conditions. 2. 2. Starvation caused a decrease in protein concentration in the hemolymph. When the hemolymph protein concentration was expressed per mg dry wt. the starved insects had a significantly higher value than the controls in most cases. This might indicate that the hemolymph protein concentration is maintained at the expense of tissue mass or that the breakdown products of the tissues accumulate in the hemolymph. 3. 3. The mammalian carcinogen, 9.10-dimethyl-1,2-benzanthracene (DMB), did not have any effect on the hemolymph protein. 4. 4. We did not show any stimulation of hemolymph protein production in the females due to the presence of males as Highnam & Lusis (1962) showed for Schistocerca gregaria.