A radioimmunoassay for human growth hormone using activated charcoal is described and its precision, accuracy, and sensitivity are defined. Results are presented for growth hormone measurements in plasma obtained during hypoglycaemia induced with insulin in patients of short stature and during glucose tolerance tests in patients with acromegaly. The method was used to measure growth hormone concentrations in cerebrospinal fluid and in extracts of pituitary tumours. No growth hormone was detected in the cerebrospinal fluid of patients without acromegaly. In patients with acromegaly, the concentration of growth hormone in cerebrospinal fluid was measurable and was considerably elevated in one patient with extrasellar extension of a pituitary tumour. Extracts of chromophobe pituitary tumours contained very small concentrations of growth hormone. In extracts of pituitary tumours removed from acromegalic patients, concentrations fell either below or within the normal range.