Plasma fibronectin (FN) concentrations were measured by an immunonephelometric method in cord blood from 76 healthy newborns (45 full term and 31 preterm), in 181 healthy children, from 1 month-15 years old, divided by age into seven groups and in 81 control adults. FN levels in newborns were 35% of those of adults. There was no difference between plasma FN in term and preterm (gestational age greater than 30 weeks) infants. Preterm newborns below the gestational age of 30 weeks showed particularly low FN levels. No clear correlation was seen with birth weight. In the child population, a strong increase in FN values was noted in the first 6 months of life, rising to 65% of the normal adult values. No further increase occurred until puberty.