A model was proposed for formation of a gel layer of biopolymer molecules at the surface of an ultrafiltration membrane under transitional conditions. It was shown that the onset of formation of this layer practically coincides with the onset of the ultrafiltration process. The transitional problem of gel layer formation is solved under the assumption that the hydraulic resistance of the gel layer significantly exceeds the membrane resistance. It is shown that in the initial stage the thickness of the gel layer increases in proportion to the square root of time, the time required to reach the steady-state thickness is calculated, and this thickness is determined as a function of pressure, concentration, and intensity of agitation of the bulk of the solution.