We present low-temperature Stark measurements on the core light-harvesting complex 1 (LH1) of purple bacteria and the B820 subunit derived from LH1, which is a protein bound Bchl a dimer. It was found that the B820 dimer exhibits only a small Stark signal dominated by a difference dipole moment between ground and excited states, Delta mu congruent to 1.4 D/f. The B820 complex can be reassociated to form LH1-like (B873) complexes, and this aggregation process induces a dramatic increase in the Stark parameters; Delta mu congruent to 3.7 D/f and Tr(Delta alpha) congruent to 1300-1800 Angstrom(3)/f(2). No significant differences were found between the B873 complex and the native LH1 antenna. The electrooptic properties of LH1 are compared to those of the special pair of the reaction center and the peripheral antenna complex, LH2, and discussed in the context of the ringlike structures observed for bacterial light-harvesting complexes. It is argued that the strong Stark signal of LH1 arises from mixing of charge transfer states with the exciton states of closely interacting pigments, the smallest possible unit being a Bchl a dimer. The absence of a strong Stark signal in B820 is most likely due to a small structural rearrangement of the protein bound dimer and the loss of interactions with neighboring pigments compared to the case of LH1.