For an explanation of ultrafast spectroscopic data observed at the photosynthetic antenna complex LHC-II of higher plants, a density matrix theory is presented. It accounts for the dissipative exciton motion among the various chlorophyll molecules in the LHC-II and enables one to simulate the time-resolved pump-probe experiments of previous work in the literature. In order to model exciton annihilation effects appearing at higher pump-beam intensities, standard exciton theory is extended to the inclusion of a second higher excited singlet state and the internal conversion process from this state to the first excited singlet state. Concentrating on a heterodimer model of the LHC-II, the approach reproduces quite well the observed ultrafast pump-intensity dependent differential absorption.