The nonlinearity parameter B/A for several biological materials was determined by measuring the amplitude of the second harmonic pressure as a function of distance from the source and the amplitude of the fundamental at the source. The B/A values for bovine serum albumin and haemoglobin solutions were found to increase approximately linearly with solution concentration. Blood and homogenized liver exhibit a B/A value similar to a haemoglobin solution of the same dry weight, but whole liver has a greater B/A value.