Sequential measurements of plasma alkaline phosphatase activity were made in 84 pre-term infants delivered before 38 weeks gestation. In 67% of infants, without evidence of rickets but in whom three or more measurements were made and the peak activity was less than 10 times the adult reference range, the activity rose to a peak and declined to previous levels, or lower. The time taken for these changes varied greatly. The range of values was inspected at each week of age, and in all but six cases the level did not exceed 10 times the upper limit of the adult reference range. Three infants had radiological evidence of rickets, and of the six cases in whom higher levels were found, only one had radiological evidence of rickets; the remainder did not. It is recommended that in the latter, the high plasma alkaline phosphatase activity should be regarded as evidence of subclinical bone disease.