The aggregation of a long-chain thiacarbocyanine dye (5,5'-dichloro-3,3'-dioctadecyl-9-ethylthiacarbocyanine perchlorate) in monolayers on a subphase containing heparin has been investigated as a function of surface pressure by in situ measurement of the absorption spectra. Only the monomer band was observed at lower surface pressure, while two J aggregate bands at 648 and 666 nm were observed when the surface pressure was above 25 mN m(-1). Their intensities increased with the surface pressure. The H aggregate was observed at higher surface pressure (50 mN m(-1)). Repeated expansion and compression of the monolayer on the heparin subphase can increase the relative amount of the H aggregates. The spectral changes in the monolayers show that there are reversible and irreversible processes in the compression and expansion cycles. In LB films, the J aggregate was formed even at lower surface pressure. Only the J band at 650 mm is observed in the LB films.