In intact (type A) chloroplasts isolated from mesophyll protoplasts of maize (Zea mays L. convar. KSC 360) the flash-induced 515 nanometer absorbance change was much higher than in conventionally prepared (types B and C) chloroplasts. The 515 nanometer signal of type A chloroplasts exhibited a biphasic rise: the initial very fast rise (rise time <<1 millisecond) was followed by a slow increase of absorbance (rise time 10 to 20 milliseconds). With decreasing degree of envelope retention the slow phase disappeared. Thus the biphasic rise of the flash-induced 515 nanometer absorbance change can be regarded as an attribute of intact chloroplasts.The 515 nanometer signal of intact chloroplasts was studied at various pH values of the external medium, with various dark intervals between the flashes and at different temperatures. The absorbance change was probed with electron transport inhibitors and ionophores. The data show that the fast phase of the absorbance increase was similar in chloroplasts isolated from protoplasts and in conventional chloroplast preparations. The slow rise, which has not been hitherto recognized in isolated chloroplasts, can be due to a contribution of the proton pump to the electric field which is generated across the thylakoid membranes.