The renal capacity of acidification of urine was studied in 21 patients with hypergammaglobulinemia (polyclonal gammapathies) secondary to several conditions. The reabsorption threshold of bicarbonate was determined by Rodriguez-Soriano's technique; the acid and ammonium production was measured using Wrong and Davies' technique. Values for the bicarbonate threshold ranged from 18.5 to 29 mmol/liter of plasma bicarbonate with a mean value of 25 +/- 2.29; they were not significantly different from the control values. Urinary pH ranged from 4.6 to 5.8; ammonium excretion ranged from 18.4 to 114.5 microEq/min (mean value 63.01 +/- 25.36). Acidity values ranged from 13 to 65.05 microEq/min (mean value 33.61 +/- 19.36). Thhre was no statistical difference between these values and those obtained from the controls. We found no evidence that the acidification function of urine was limited in patients with hypergammaglobulinemia secondary to several conditions as compared with normal subjects. In the two cases in which this function was altered there was no reason why it should be attributed to hypergammaglobulinemia.