Histochemical localization of folic acid in the cells.

Smears, fresh cryotome sections (dried at 4 degrees C for 24 h), and deparaffinized sections were used for the "in situ" localization of folic acid. After irradiation with ultraviolet light, folic acid breaks down into a pteridine and a diazotizable amine. After oxidation and a brief hydrolysis, glyoxylic acid formed from the pteridine can be vizualized by a coupling agent, indicating the sites of folic acid in the cell.