An investigation of the role of phosphotungstic and phosphomolybdic acids in Mallory-like trichrome methods showed unexpectedly that, rather than acting as mordants to anionic dyes, these polyacids selectively blocked staining of all tissue components other than connective tissue fibres to Aniline Blue and other similar fibrereactive dyes. Connective tissue components were found to contain residues resembling histidine that are easily accessible to anionic dyes. Blocking towards typical anionic dyes for demonstrating plasma proteins, such as Biebrich Scarlet, was also demonstrated but was less complete. The blockade of both types of dye was labile if the staining times were extended; plasma dyes were more sensitive than fibre dyes in this respect. Histochemical reactions for tyrosine residues were blocked. In connective tissue, phosphotungstic acid did not block histidine residues demonstrable by the coupled tetrazonium reaction with previous iodination. Thus it is postulated that differential trichrome staining occurs by binding of Aniline Blue to basic residues in the connective tissue not blocked by phosphotungstic acid and subsequent replacement of the blocking agent by an anionic dye. The binding of phosphotungstic acid to both epithelium and connective tissue was demonstrated by the quenching of autofluorescence in these regions and by the reduction of the bound PTA to blue coloured products with titanium trichloride.