Abstract: A process of producing a porous polyurethane resin filter material. A first solution containing an NCO-adduct dissolved in a highly polar organic solvent is admixed into a second solution containing a highly polar organic solvent and at least one hydrazine hydrate, carbodihydrazide, piperazine and 1,4-diaminopiperazine. The first solution is admixed into the second solution over an extended period of time during which time the viscosity of the admixture is monitored and during which time the NCO pre-adduct and the hydrazine (or the like) react to form a polyurethane reaction product. During this time, the viscosity of the admixture increases. Admixture is continued with monitoring of the viscosity of the admixture until a point is reached at which the addition of even the minutest amount of the first solution into the admixture would result in instantaneous gelling thereof. At this point, the solids content of the final admixture is between 15 and 35% by weight.