A solution of recombinant interleukin-3 produced using mammalian cell culture is concentrated by batch ultrafiltration. The solution viscosity is 1.2 mPa s and the density is 1.0 g cm^-3. A battery of spiral-wound membrane modules containing a total membrane area of 250 m2 is available for the process. The feed channel height in the modules is 2.5 mm and mesh spacers are installed to improve mass transfer. The cross-flow liquid velocity is 45 cm s^-1 . The membrane molecular weight cut-off is chosen so that the permeate contains negligible interleukin. Interleukin is concentrated by a factor of 20 under gel polarisation conditions from an initial concentration of 5 units ml^-1. The diffusivity of interleukin-3 in aqueous solution is estimated as 11.5 X 10^-7 cm² s^-1 and the gelation concentration is 2000 units ml^-1. Fouling effects are negligible. What time is required to treat 15 m3 of dilute interleukin solution?