Plasma fibronectin levels and complete blood cell counts were assessed prospectively among 100 infants less than 3 months of age with the provisional diagnosis of "possible sepsis". Seven of the ten infants with culture-proved bacteremia, meningitis, or urinary tract infection had low plasma fibronectin levels as did 12 (13%) of 90 infants with superficial or no documented bacterial infection. The positive predictive value of a low plasma fibronectin level in conjunction with leukocytosis and elevated band ratio for discriminating serious bacterial infection was 71%. Normal white blood cell counts or fibronectin level alone or in combination predicted the absence of serious bacterial infection with an accuracy of at least 94%. Plasma fibronectin determination provides a useful adjunct to the complete blood cell count for the rapid evaluation of extent of illness in young infants with possible sepsis.