A simple method is described which allows fractionation into 60 μl fractions of the system obtained at density gradient isoelectric focusing in a 1.5 ml column. By measuring the pH of each fraction with a micro glass electrode, p I's of focused proteins can be evaluated with high precision. Possible biological activity may also be estimated. A method for increasing the resolution of the activity curve is discussed briefly. p I values for bovine β-lactoglobulins A and B at 25°C are reported (5.13 and 5.23, respectively).