Several hollow fiber ultrafiltration membranes have been produced from polyethersulfone-polyvinyl pyrrolidone-N-methyl-2-pyrrolidone solutions and tested. The effects of feed flow velocity through the fiber bore on pressure drop in the test fiber bundle, membrane separations for PEG solutes, and the obtainable mass transfer coefficients under the test conditions have been experimentally determined and discussed. Further, the effects of storage time and prefiltration of the fiber casting polymer solution on the performance characteristics of the resulting membranes have been studied. The viscosity of the fiber casting solution increased upon storage for a few weeks, and it decreased after the solution was filtered prior to use in fiber production. Storage time and filtration treatment of the fiber casting solution had significant effects on the morphology of the resulting membranes. Casting solution of longer storage time and without filtration pretreatment produced smaller size pores on the membrane bore-side skin layer, and a larger number of such pores and/or a thinner skin layer in the resulting membrane. On the basis of these results, the need for standardizing the fiber producing conditions and fiber testing procedure is pointed out, and a few suggestions are made for such standardization.