Optimal functioning of the immune system is necessary for the host to be capable of mounting a sufficient immune response, especially in times of sickness and injury. Acute bouts of starvation may compromise immune function, and subsequently lead to increased susceptibility to infection. Immunocompetence has been suggested as a functional indicator of nutritional status as the function of the immune system relies upon nutrient dependent metabolic pathways and the provision of adequate nutrient substrates to synthesize its components. The sensitivity of monocyte phagocytic activity, major histocompatibility complex (MHC) class II expression, and fibronectin concentration were studied in 23 healthy cats during a 7 day period without food followed by a 7 day refeeding period. Blood samples were obtained for plasma fibronectin analysis and immune cell function tests on days 0, 4, 7, 11, and 14. A turbidimetric immunoassay was used for determination of plasma fibronectin concentration. Monocyte phagocytosis and MHC class II expression were measured using flow cytometric techniques. Weight, lymphocyte number, percent lymphocytes, white blood cell number, and serum albumin concentration were monitored throughout the study. Phagocytic activity, MHC class II expression, weight, lymphocyte number, percent lymphocytes, and white blood cell (WBC) number, decreased significantly (p<0.05) during the starvation period. Fibronectin concentration increased significantly (p<0.05) by day 4 of starvation. During refeeding there was a significant increase (p<0.05) in MHC class II expression, fibronectin concentration, weight, lymphocyte number, percent lymphocytes, and white blood cell number. Phagocytic activity decreased significantly (p<0.05) by day 11 of refeeding. Pearsons correlation analysis revealed a positive correlation (p<0.05, r=.2682) between weight change and phagocytosis. There was a positive correlation (p<0.05, r=.3588) between monocyte number and MHC class II expression, and between monocyte number and WBC number (p<0.05, r=.3506). Results indicate that maintenance of immune function is dependent upon the provision of continuous nutritional intake by the host. Plasma fibronectin, monocyte phagocytosis, MHC class II expression, and other immunological measures of health status were sensitive to acute alterations of nutritional intake and subsequent refeeding. Both phagocytic activity and MHC class II expression were found to be reliable indicators of nutritional status during acute nutritional deprivation. These data suggest that short periods of food deprivation may significantly decrease immune response.