The interaction between the light-harvesting antenna (LH1) and the reaction center (RC) in the photosynthetic purple bacteria Rhodobacter sphaeroides R26 and strain 2.4.1 was studied with low-temperature (1.5 K) absorbance-detected magnetic resonance (ADMR) experiments combined with computer simulations. The triplet-minus-singlet (T-S) spectra of chromatophores of Rb. sphaeroides R26 and strain 2.4.1 show an extra positive component at approximately 884 and 887 nm, respectively, and an extra negative component at 897 and 901 nm, respectively, when compared to RCs of Rb. sphaeroides R26 and strain RCO1. Computer simulations using a simple model of the RC encircled by the antenna of LH1 show that the dipole strength and energies of the second-lowest exciton component of the antenna and the RC change when the coupling term between the RC and antenna is included in the Hamiltonian of the system. Thus the difference in the shape of the ADMR-detected T-S spectra of isolated RCs (or strain RCO1 chromatophores) and chromatophores is plausibly caused by a change in the excitonic interaction between the RC and antenna.