The time-dependent filtration pressure curves of cell suspensions pumped through 5 microns polycarbonate filters at a constant flow rate were analyzed with the aid of a theoretical model developed in an accompanying paper. The cell suspensions contained mixtures of erythrocytes and leukocytes, the concentrations of which were systematically varied. The pressure-time (P-t) curves generally showed multiphasic components. Following the attainment of a quasi-steady state level, the pressure rose first rapidly and then more slowly. The rates of pressure rise in the fast and slow phases were normalized by using the steady state pressure reading (PO) obtained with Ringer solution at the same flow rate, and are designated k1 and k2, respectively. Both k1 and k2 increased with rising concentrations of leukocytes, [WBC], or erythrocytes, [RBC]. [WBC] is 700-1000 times more effective than [RBC] in affecting k1 and k2. k1 is related to the dynamic plugging and unplugging of filter pores, primarily by leukocytes. k2 is attributable to the "permanent" plugging of filter pores, again predominantly by leukocytes. The experimental P-t curves can be fitted with the theoretical model by using appropriate constants for leukocyte plugging. The results indicate that nearly 2/3 of the entering leukocytes cause transient plugging of pores, with an unplugging rate of 4.1 percent/sec/unit pressure rise, and that approximately 2.2 percent of the entering leukocytes are "permanently" lodged. These results underscore the important role of leukocytes in determining the later phase of the P-t curve and support the concept that leukocyte plugging may have pathophysiological significance in causing microvascular occlusion in disease states.