Enzyme urease is extracted from the discarded seeds of pumpkin. Urease was purified to apparent homogeneity (5.2 fold) by heat treatment at $48;{pm};1^{circ}C$ and gel filtration through Sephadex G-200. Effect of model metal ions on the activity of the homogeneous enzyme preparation (sp. activity 353 U/mg protein, $A_{280}/A_{260};=;1.12$) of soluble as well as immobilized enzyme was investigated. The soluble and immobilized urease has been used for the quantitative estimation of general water pollution with heavy metal ions like $Hg^{2+}$, $Cu^{2+}$, $Cd^{2+}$, and $Co^{2+}$. The measurements of the urease residual activity have been carried out in tris-acetate buffer after pre-incubation of model metal salt. The inhibition was found to be biphasic with an initial rapid loss of activity and remainder in slow phase of $10{sim}15$ min. The immobilization was done in 3.5% alginate beads leading to 86% of entrapment. There was no leaching of the enzyme over a period of 15 days at $4^{circ}C$. The beads were fairly stable up to $50^{circ}C$ and exhibited activity even at $-10^{circ}C$. The inhibition by these ions was non-competitive and irreversible, hence could not be restored by dialysis. Based on the values of inhibition constant $K_i$ the heavy-metal ions were found to inhibit urease in the following order $Hg^{2+};>;Cu^{2+};>;Cd^{2+};>;Co^{2+}$.