Methyl Red H-star aggregate of predetermined size is successfully synthesized from the DNA conjugate involving multiple Methyl Red moieties in sequence. In the single stranded state, hypsochromicity monotonically increases with the number of incorporated dyes: the peak maximum of the conjugate involving six Methyl Reds appears at 415 nm, and the shift is as great as 69 nm (3435 cm(-1)) with respect to the monomeric transition. This large hypsochromicity accompanied by the narrowing of the band clearly demonstrates that H-star aggregate is formed in the single strand. H-star aggregation is further promoted at higher ionic strength. Upon addition of complementary DNA below the T-m, however, this H-star band disappears and a new peak appears at 448 nm, indicating that aggregated structure is changed by the duplex formation. This spectral change is completely reversible so that the H-star band at 415 nm appears again above T-m. Thus, aggregated structure can be reversibly controlled by the formation and dissociation of the DNA duplex.