METHODS Basal medium. A modification of the folic acid assay medium described by Waters & Mollin (1961) was used. Excess folic acid (20 ng/ml) was included in the medium but purines and pyrimidines were omitted. Buffering capacity was increased by addition of 0.2 M- Sorensen's phosphate buffer, pH 6.6 (500 ml/l), and acid-hydrolysed vitamin-free casein (Difco) was used instead of enzyme-hydrolysed casein. A slight precipitate appeared on autoclaving but cleared on cooling. Lactobacillus casei ATCC 7469 was maintained on agar slopes prepared by adding a mixture of 5 mg/l of each of adenine sulphate, guanine hydrochloride and uracil to the single- strength medium solidified with 2 % (w/v) Bacto-Agar (Difco). A stock liquid culture in I o ml of single-strength medium with added purines and pyrimidines was subcultured daily. The inoculum was prepared by washing the bacteria from a 24 h broth culture twice with sterile NaCl (0.9 %, w/v) and finally resuspending in NaCl to E560 of 0.05 units. Standard solutions. Imuran azathioprine for injection (Burroughs Wellcome & Co., Dartford, Kent) in vials containing the equivalent of 50 mg free drug as the sodium salt was used. Mercaptopurine B.P. (6-MP; IOO mesh B.S.S.) was obtained from Burroughs Well- come & Co. Due to the limited solubility of both drugs in water, 50 mg of each was dissolved in 20 ml of 0.1 M-NaOH and then diluted immediately to 500 ml with distilled de-ionized water to give ~oopg druglml. Stock aqueous solutions of azathioprine and 6-MP in the range 20 to 2000 ng/ml were prepared. These solutions were stored at 4 "C and used within 24 h of preparation.