Tau (5 μM) was incubated (37°C for 18 h) with arachidonic acid inducer (50 μM) in the presence of varying concentrations of selective compounds Thiazine red R (1a), the aniline 4a, Crystal violet (4b), and the indolinone 6a. Each compound was tested at AC50 and four times the AC50 values as determined by the fluorescence assay, and then examined for tau fibrillization by electron microscopy. The lowest concentrations (black bars) of 1a, 4a, 4b, and 6a were 2.1 μM, 1.6 μM, 0.18 μM, and 1.0 μM, respectively. Highest concentrations (white bars) of 1a, 4a, 4b, and 6a were 8.4 μM, 6.4 μM, 0.72 μM, and 4.0 μM, respectively. Filament length (Γf) is reported as the percent filament length observed in the presence of DMSO vehicle alone (labeled as C, hatched bar), which is defined as 100%. Error bars indicate S.D. Statistical significance of differences in total filament length was assessed by Student’s t test (*, p < 0.05).