Usually cell viability (CV) after PDT is observed in methods based on metabolic parameters (MTT assay) or dye exclusion tests [trypan-blue (TBT)]. Although these tests are accurate in the determination of cell death, functional impairment can not be easily detected. We examined the effects of PDT on human dermal fibroblasts (DF) to determine whether collagen gel contraction (CGC) can be inhibited and whether it correlates well with CV as measured by the TBT. Five-aminolevulinic acid (5-ALA) (1, 5, 10 (mu) g/ml) was added to DF cultured in dishes. Twenty-four hours later, a collagen type I solution was added and irradiation with a tunable dye laser (630 nm, 100 mW/cm2, 80 J/cm2) was performed. After 24 h, diameters of contracted gels and TBT were measured. As a result, PDT with 5-ALA inhibited CGC and affected CV as a function of dose level. Consequently, CGC data allows a better estimation of the threshold 5-ALA dose affecting the cells.