A direct, rapid (<3h), and simple technique is described for assaying human plasminogen by affinity chromatography. Plasma is passed over a column of Sepharose—butyl aminobenzoate. The unretarded proteins, including plasmin, are washed out with phosphate—saline, pH 7.4, buffer. The bound plasminogen is released by adding lysine, 0.1 mol/liter, to the buffer and the plasminogen concentration in the eluate is determined by absorbance measurements at 280 nm. A value of 195 ± 10 mg/liter was found for pooled donor plasma from normal individuals. Twenty-one normal women, age 21 to 46, had an average value of 194 ± 38 mg/liter, while 14 pregnant women in the third trimester had an average value of 250 ± 55 mg/liter. The range in 30 hospital patients with various ailments was 122 to 265 mg/liter.