The photodynamic bactericidal effect of the photoactive dyes acriflavine neutral, rose bengal, phloxine B, and malachite green (oxalate salt) at concentrations of 5 to 5,000 microg/ml against two gram-negative strains (Escherichia coli LJH 128 and Salmonella Typhimurium C1058), two gram-positive strains (Bacillus sp. C578 and Listeria monocytogenes LJH 375), and yeast (Saccharomyces cerevisiae C1172) was investigated. Incubation of the investigated bacteria with acriflavine neutral under illumination resulted in a significant reduction in cell numbers compared with dark incubation. Rose bengal caused a significant killing effect for bacteria incubated both in the dark and under illumination. Malachite green was active against gram-positive bacteria under illumination and did not affect gram-negative bacteria or yeasts. Incubation with phloxine B resulted in a significant decline in cell numbers for gram-positive bacteria, both in the dark and under illumination; gram-negative bacteria and yeasts were unaffected. Conjugation of rose bengal and phloxine B with poly(vinyl amine) resulted in an enhanced bactericidal effect during both dark and light incubation. This was explained by electrostatic interaction of the polymer with the cell surface, which resulted in closer contact of the photoactive dye and cell. No killing effect was observed for yeasts incubated with dye conjugates. Filter paper treated with dye-poly(vinyl amine) conjugates showed high photodynamic bactericidal activity against the bacterial strains, but not against the yeasts. The extent of bacterial killing depended on the nature and concentration of the dye conjugate and the type of microorganism. The presented data suggest that a photodynamic approach for constructing "self-decontaminating" materials has potential.