To the Editor: In patients receiving methyldopa (Aldomet), a strong color reaction has regularly been observed when the urine is tested for porphobilinogen with the modified Watson-Schwartz test.1 The approximately equal distribution of the pink or red color between the aqueous and butanol layers distinguishes this reaction from that of porphobilinogen. Urine samples from 57 patients receiving methyldopa were tested. Patients taking at least 750 mg of methyldopa daily consistently showed this color reaction in the urine. Two of 57 patients had a positive Hoesch test2 but had normal amounts of porphobilinogen in the urine on quantitative measurement.3 None. No extract is available for articles shorter than 400 words.