We report the results of relative difference absorption measurements for the LH2 antenna of Rhodobacter sphaeroides and the B820 subunit of Rhodospirillum I rubrum at room temperature. It is shown that significant differences between shapes and amplitudes of photoinduced, absorption changes reflect a different degree of exciton delocalization in the intact antenna compared with the dimeric subunit, Using the exciton model in the presence of static disorder, we have obtained a consistent and quantitative fit of the amplitudes and shapes of the pump-probe spectra of the LH2 antenna and the B820 subunit. We estimate that the nearest-neighbors interaction energy in the antenna is about 400 cm(-1) and the diagonal disorder is about 450 cm(-1). For these values the coherence length (FWHM) of the steady-state exciton wavepacket corresponds to 5 BChl molecules at room temperature. The amplitude of the difference absorption reflects a cooperative behavior within at least 12 BChls of the B850 antenna. The calculations suggest that the dimeric subunit is characterized by a decrease of the interaction energy to 300 cm(-1) together with an increase of the disorder value to about 600.