The soluble proteins (chromogranins) of bovine chromaffin granules have been studied by micro‐osmometry with semi‐permeable membranes (UM2, PM10 and PM30 with cut‐offs>1,>10 and>30 kD, respectively) at I =0.15 and pH 5–8 for protein concentrations up to 20 mg ml‐1. After lysis of chromaffin granules in phosphate buffer pH 6, the released chromogranins behaved as aggregating solutes, consistent with an inconspicuous osmotic pressure contribution from the chromogranins at the protein concentration of the intact granules. Thus, in the presence of phosphate about 90% of the molecules behaved as colloids withMr=30,300 atc =o. After lysis in phosphate‐free buffers the chromogranins behaved as highly non‐ideal solutes in a manner which was incompatible with isotonicity at the protein concentration of the intact granules. About two‐thirds of the molecules in the lysates in Na‐succinate pH 5–6 and K‐acetate pH 6 exhibitedMr= 66,000 and 79,000, respectively. In dilute solutions (<12mg protein ml‐1) and ATP/protein ratios corresponding to those in the intact granules, the UM2 pressures were markedly increased, indicating release of polypeptides withMr2000–3000 from aggregates. CaCl2was without specific effect on the colloid osmotic pressures but reduced the ATP‐dependent increase in pressure, suggesting release of molecules twice the size of those released by ATP alone. A model is presented for the contribution of the chromogranins to osmotic pressure regulation in the bovine adrenomedullary cat