Background and Objectives The difference in the photobactericidal efficacy of methylene blue and toluidine blue against gram‐negative bacteria may result from their primary reaction with lipopolysaccharides (LPS) of the outer bacterial membrane. The aim of the present study was to compare the reactivity of these dyes with LPS extracted from different gram‐negative bacteria. Study Design/Materials and Methods The interactions of methylene blue and toluidine blue with LPS from Escherichia coli (E. coli), Pseudomonas aeruginosa (P. aeruginosa), Klebsiella pneumoniae (K. pneumoniae), and Serratia marcescens (S. marcescens) were studied spectrophotometrically in 0.45% saline. The dyes were used at the concentration of 10 μM. The concentrations of LPS ranged from 5–100 μg/ml. Results Methylene blue and toluidine blue enter into a metachromatic reaction with the LPS resulting the in generation of dimers of methylene blue and higher aggregates of toluidine blue. The more significant hypochromic and hypsochromic effects in the reaction of the latter with LPS indicate a greater metachromatic efficacy of toluidine blue than methylene blue. The equilibrium constants of the metachromatic complex between toluidine blue and different LPS were calculated. The spectrophotometric titration of LPS with the dyes was used to estimate the equivalent weight of LPS. Conclusions Toluidine blue interacts with LPS more significantly than methylene blue in vitro. This may be one of the main factors determining its greater photobactericidal efficacy against gram‐negative bacteria. Lasers Surg. Med. 33:311–319, 2003. © 2003 Wiley‐Liss, Inc.