A possible interaction between zinc sulphate and tetracycline or doxycycline was examined in a cross-over study in seven volunteers. A single dose of zinc sulphate (45 mg Zn++) was given simultaneously with tetracycline hydrochloride (500 mg) or doxycyline chloride (200 mg). The serum concentration of tetracycline, the area under the serum tetracycline concentration-time curve and the excretion of tetracycline in urine were reduced by about 30% (p less than 0.05) from the respective control values. The absorption of doxycycline was not influenced significantly by zinc sulphate. The clinical significance of the zinc-tetracycline interaction seems to be of limited importance.