We set out to design a simple test, derived from the Dubowitz gestational assessment, to differentiate immature from mature newborn infants. An initial group of 196 infants of known gestation was studied to determine the two criteria with the most discriminant value. It was found that the clearest cut-off for most criteria occurred at 36 rather than at 37 weeks gestation, and that the best criteria were breast size and plantar creases. The usefulness of this pair of criteria was then assessed in a second group of 120 babies weighing 1750-2200 g where a full 21-point Dubowitz assessment had been used to estimate gestational age. Based on this assessment, babies were categorized as "true immature" (less than 36 weeks) or "true mature". In this low-birthweight group, if maturity (36 weeks or more) was diagnosed when both the breast nodule and the plantar creases scored 2 or more on Dubowitz rating, then 79% of "true immature" and 81% of "true mature" infants were correctly classified. Thus, in low-birthweight Melanesian infants, a baby with indentations on the sole (or red marks on the posterior half) and breast tissue of diameter 0.5 cm or more on one or both sides is likely to be mature. If one or both signs is absent, immaturity is likely. This simple assessment of maturity would be practical and easy to teach in primary health care in developing countries.