Although it is well known that growth hormone may influence protein metabolism, few investigations have been undertaken on serum protein levels in acromegaly; recently hypoglobulinemia has been observed in this endocrine disorder. In 28 acromegalic patients and 56 control subjects sera have been analyzed for total protein determination and for electrophoretic protein separation. The results have shown that in acromegalic there is a slight but statistically highly significant decrease of serum total protein (mean +/- SE 6.66 +/- 0.07 g/100 ml vs 7.10 +/- 0.07 g/100 ml), albumin, alpha 1 and alpha 2-globulins. Since growth hormone stimulates protein synthesis, the pathophysiological significance of hypoproteinemia of acromegaly is at present obscure.