Abstract: 1. Bovine serum albumin stimulates the DCIP photoreduction activity of lettuce chloroplasts which has been treated with trypsin. When these chloroplast preparations were washed by tricine buffer such "reversible action" can still be obtained. It is possible that bovine serum albumin may be incorporated into trypsin destroyed site of the membrane. 2. Trypsin-induced CCCP inhibitory effect on DCIP photoreduction activity is reversed by bovine serum albumin. 3. Bovine serum albumin partially reverses the trypsin-induced unstacking of lettuce chloroplast membranes. 4. After trypsin digestion, there are absorbance decreases around 500–640 nm. Bovine serum albumin has no effect on these absorbance decreases. It is concluded that the membrane-bound proteins responsible for different functions of chloroplast are heterogeneous. The results also show that there are gate and channel near the position of PSⅡ on chloroplast membrane.