Rennet is a substance containing rennin (an enzyme in the gastric juice that coagulates or curdles milk.), obtained from the stomach of a calf or other ruminant, used for curdling milk in making cheese and junket. Rennet should be avoided because it is extracted from animals which have not been slaughtered according to Islamic Law. However, if the source animal from which rennet was extracted was halal and slaughtered according to Islamic Law, then there is no problem.