Urease from the seeds of watermelon (Citrullus vulgaris) was purified to apparent homogeneity, using two acetone fractionation steps, heat treatment at 48 degrees C and gel filtration through Sephadex G-200. Effect of acetohydroxamic acid (AHA) on the activity of the homogeneous enzyme preparation (sp. act. 3000 +/- 550U/mg protein) was investigated. AHA exhibited a concentration-dependent inhibition both in the presence and absence of the substrate. The inhibition was uncompetitive and the Ki was 2.5 mM. Binding of AHA with the enzyme was reversible, as 63% activity could be restored by dialysis. Time-dependent inhibition revealed a monophasic inhibition of the activity. Addition of beta-mercaptoethanol (ME) gradually abolished the inhibition. Pre-treatment of native enzyme with 8.0 mM ME for 5 min at 30 degrees C exhibited protection against AHA-induced inhibition. The significance of these observations is discussed.