Abstract 1. 1. Under all experimental conditions the protein concentration of the hemolymph rose during the first half of the fifth stadium. During the second half of the fifth stadium the pattern is not clear. The concentration increases during the first 4 days of the adult stage and remains high until at least the fourteenth day post-molt. 2. 2. In 1 lambda, non-pooled samples, a significant difference for mean protein concentration values was found between the following groups: (a) male and female adults, 5 days post-molt; (b) male and female adults, 10 days post-molt; and (c) the 10 per cent and 50 per cent fifth instar. 3. 3. The concentration values reported can be used in comparative studies of protein concentration in insects of known age, sex, developmental stage and nutritional state; for studies on the hemolymph from the milkweed bug under various experimental conditions; and for comparison with other animals.