The dearth of plasma alpha fetoprotein reference ranges for preterm infants often impairs the clinical interpretation of plasma alpha fetoprotein data collected from ill babies. This study tested our hypothesis that meaningful plasma reference ranges could be established for preterm infants by a simple correction of patient age at sampling date for gestational age deficit at birth. Using a modified radioimmunoassay kit method, determinations of alpha fetoprotein were performed on capillary and venous blood samples collected from 56 babies aged from birth to 5 months with gestational ages ranging from 26 weeks to 43 weeks. Unmodified plasma alpha fetoprotein values were grouped according to patient age and examined statistically using established normal theory methods, but these yielded excessively wide reference intervals and non-Gaussian distribution parameters. Acceptable reference ranges were derived using logarithmic transformation of plasma alpha fetoprotein values and rearrangement against patient age corrected for gestational age deficit. These provisional reference ranges for plasma alpha fetoprotein in preterm (and term) infants are applied to groups of previously meaningless alpha fetoprotein results and used to test the potential usefulness of plasma alpha fetoprotein determination as a diagnostic marker in biliary atresia, hepatitis, and yolk sac derived tumours.