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We measured hematocrit and serum albumin concentration at birth and red blood cell and plasma volume soon after birth in prematurely born infants with erythroblastosis fetalis of varying severity and examined the realtionships between these variables and the presence and severity of hydrops fetalis. Blood volumes in most of these infants were similar to the established normals for newborn infants without erythroblastosis. There was no simple association between blood volume and the presence of hydrops. Nonhydropic and severely hydropic infants had, on the average, similar and normal blood volumes, while mildly hydropic infants had low blood volumes. Anemia correlated fairly well with severity of hydrops but almost a quarter of the infants with severe hydrops were only mildly anemic. Red blood cell volume decreased and plasma volume increased proportionally with the degree of anemia at birth. Thus, hydropic infants with severe anemia had large plasma volumes while those with milder anemia did not. On the other hand, hypoalbuminemia was common and correlated closely with severity of hydrops. We suggest that hydrops results at least in part from low plasma colloid osmotic pressure due to hypoalbuminemia.