Chloroplast preparations capable of rapid rates of photosynthesis contain large numbers of intact (Class A) chloroplasts and smaller numbers of ruptured (Class C) chloroplasts. It is suggested that there is also a third or intermediate class in which the envelope has re-sealed following rupture and release of stromal protein. The number of re-sealed chloroplasts seems to be lowest in preparations containing the largest proportion of Class A chloroplasts. Assays for intactness based on the ability of Class C chloroplasts to utilize ferricyanide as a Hill oxidant are quick and convenient but do not distinguish between Class A and re-sealed chloroplasts. If it is assumed that there is an inverse linear relationship between percentage intactness and ability to react with ferricyanide, such assays over-estimate the proportion of intact chloroplasts and, therefore, underestimate their photosynthetic activity and protein content when these are expressed on a chlorophyll basis.