Abstract: The technological characteristics of pretreatment in purification process of antibiotic fermentation broth are studied in this paper.The effects of some factors on filtration rate and on concentration are discussed.Spiramycin (SPM) and Penicillin (Pn)are studied in details as examples. It is discovered in the experimentsthat the concentration of SPM or Pn in the filtrate varied with the pH value. The concentration reaches a maximum when pH=5.5.At the same time, a solution equilibrium of protein is present. The protein will denature on the condition of excess acid or base, and its concentration in solution reaches a minimum when pH=6.0—6.5. There is also an adsorption equilibrium of SPM on protein precipitate and other solids.The recoveryof filtrate is influenced by many extraneous materials because of rivalry between them.A rational explanationis given by application of modified Langmuir adsorption theory based on the experimental results.The theoretical model is developed and mathematical formulas are derived.