The interaction of meso-tetra(4-sulphonatophenyl) porphyrin (TPPS4) with bovine serum albumin (BSA) in acid medium was investigated using optical absorption spectroscopy. Changes in the absorption spectra indicate that a mechanism of interaction between TPPS4 and BSA is in a complex manner dependent on: (a) the ratio between monomeric and aggregated species in a homogenous solution of TPPS4, (b) the molar ratio of the TPPS4-BSA mixture, and (c) the initial state of protein solution. When different forms of TPPS4 exist in solution, small relative concentrations of BSA favour J-aggregation; however, higher concentrations may lead to the disruption of J-aggregates. If J-aggregates are preformed in the solution, adding of protein does not disrupt these aggregates.