Natural and purified proteins were split by crystallized pepsin. The break down of substrates was followed by a turbidimetric method. pH optima were found from 1.5 to 3.8. Only unpurified egg albumin had two optima, crystallin had one in the acid range. Ovomucoid not being split by pepsin, the second peak at pH 3.79 is referred to conalbumin. While the disintegration of caseinogen becomes slow with decreasing acidity, this is not true for casein (paracasein) which is well split up to pH 3.28 (cows casein) or 3,73 (womens casein). Nevertheless, the optimum is in the range of 1.5–2.0, like most proteins. Above this limit, the following optima were found: fibrinogen 2.93, thrombin 2.78, edestin 3.08, crystallized hemoglobin 3.19, conalbumin 3.79.