SUMMARY: Non-specific factors in urine, measuring as apparent growth hormone, were detected when the radioimmunoassay for plasma growth hormone was applied to urine, dilute urine and fractions therefrom. Varying concentrations of salt and urea were shown to mimic the effect of growth hormone and, like the urine inhibition, were lost on dialysis. Urine from normal subjects and from a hypopituitary dwarf treated with growth hormone, when concentrated, desalted and fractionated by gel filtration gave no evidence for the presence of growth hormone. From recovery experiments it was concluded that growth hormone in normal urine is present at concentrations below 0·1 μg./I. An inhibitor with the immunological characteristics of growth hormone was obtained from a fraction of concentrated urine from an acromegalic patient and was equivalent to 0·24 μg. growth hormone/24 hr. urine.