A series of squaraine dyes are found to be in H- or J-aggregates and are almost nonfluorescent in aqueous solution. Upon addition of bovine serum albumin (BSA), however, the fluorescence intensity (at lambda(em) approximate to 690 nm) increases by a factor of up to 200. Transformation of the dye molecules from the aggregates to the monomeric species appears to be responsible for the large fluorescence turn-on. While both H- and J-aggregates contribute to the observed fluorescence turn-on, the former appears to play a more important role. Electrophoresis imaging shows that these probes are good BSA indicators.