Marked alterations of plasma protein distribution occurred in white-footed mice (Peromyscus leucopus) infested with botfly (Cuterebra angustifrons) larvae. Reductions in the albumin-to-globulin ratio of animals infested with botfly larvae were correlated significantly with the number of larvae present. Development of the Cuterebra larva in the host was accompanied by progressive depression in the A/G. Correspondingly, recovery in the A/G to the normal values occurred 8 days following loss of the bot. Ingestion of albumin by the bot was proposed as a partial explanation for the reduction in A/G in this type of dipteran parasitism. Electrophoretic patterns of serum and plasma proteins are altered in a variety of pathological conditions (Bier, 1959; Sobotka and Stewart, 1958; and Wuhrmann and Wunderly, 1960). For most abnormal conditions studied, a consistent change has been a reduction in the serum albumin, concurrent with a relative or absolute increase in the globulin fraction(s). Alterations in several mechanisms could effect these changes. Some of the most obvious are a decrease of albumin synthesis in the liver, failure of renal reabsorption or an increased rate of metabolism of albumin, and loss of albumin following increased capillary permeability (Shekarchi and Makinodan, 1960). In some conditions, other mechanisms in addition to these may play a role in bringing about a reduction in the albumin-to-globulin ratio (A/G) below the normal range found for a particular group of animals. However, with some notable exceptions, especially in regard to irradiation damage (Glinos, 1963; Muntz et al., 1949), few attempts have been made to define the factors responsible for an imbalance in serum proteins in specific disorders other than for the most obvious such as liver or kidney malfunction. Also, little attention has been directed toward correlating the degree of depression of the A/G with the severity of the condition or to describing the rate at which this ratio approaches the normal range of values in animals recovering from the acute or active stages of a given infection. Received for publication 2 April 1965. * Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corporation. t Present address: Bowman Gray School of Medicine, Winston-Salem, North Carolina. In 1963, plasma electrophoretic studies at Oak Ridge National Laboratory on a native white-footed mouse (Peromyscus leucopus) population indicated a significant reduction in the A/G for those animals infested with larvae of a botfly, Cuterebra angustifrons1 (Dunaway et al., 1964). This condition of skin myiasis was studied more extensively in the summer of 1964. An attempt was made to correlate the depression in A/G with the stages of development of the larva and to the number of botfly larvae infesting the animals. The manner in which the plasma protein patterns varied after the larvae emerged from their hosts was also studied. In addition, observations were made on the protein patterns of the excreta and body fluid of the bots in an attempt to gain some insight into the mechanism responsible for the reduced A/G in affected mammals. MATERIALS AND METHODS Thirty-four adult Peromyscus leucopus (18 with one or more bot larvae) were livetrapped during the summer of 1964 for use in this experiment. The mice were caged singly in the laboratory and were maintained on an ad libitum diet of Purina Laboratory Chow and water throughout the experiment. Each animal was bled 1 day after being brought into the laboratory. The P. leucopus containing bots were again bled after the mature bots had left their hosts. The freshly dropped bots were weighed. Control white-footed mice were bled on a similar schedule. Blood samples for analyses of plasma protein patterns were obtained from the caudal veins of the animals. Samples of body fluid and excreta C. W. Sabrosky of the U. S. National Museum provided identification of a reared adult botfly. Fourteen adult flies of the same species were reared from larvae infesting mice utilized in this study.