Protein recovery from a bacterial lysate was accomplished using crossflow microfiltration with rapid backpulsing. The net flux with backpulsing was found to increase with increasing forward and backpulse pressures, increase to a maximum and then decrease with increasing forward filtration time between backpulses (or decreasing backpulse frequency), increase weakly with increasing shear rate, and decrease strongly with increasing concentration in the feed. Variation of the operating conditions for a dilute feed of 0.0025 g cell debris/g suspension on a wet cell mass basis yielded net flux values as high as 4.5 X 10(-3) cm/s (160 l/m(2) h) with backpulsing, compared to a steady-state value of 4.0 X 10(-4) cm/s (14 l/m(2) h) for similar conditions without backpulsing. The optimal backpulse frequency was found to be very high, about 2.5 times per second, for the minimum backpulse duration of 0.09 s. However, the performance of the backpulsing operation declined with increasing concentration of cell debris, with no flux improvement achieved for concentrations greater than 0.01 g cell debris/g suspension. Nevertheless, 100% protein transmission with backpulsing was achieved for all conditions investigated, compared to an average value of 60% transmission in the absence of backpulsing. Thus, rapid backpulsing is an effective method for the recovery of protein from dilute cell debris.