West Yorkshire Police received a flood of emergency calls from people in the Leeds area of Great Britain who were mailed 60,000 unmarked packages containing a granulated water-saving substance.
One resident put on rubber gloves and opened the package in his garden when he heard a rustling in the envelope.
The envelopes contained a "Save-a-Flush" device that reduces the amount of water used when the toilet is flushed.
The mailing was part of Yorkshire Water's ongoing campaign to encourage people to conserve water in the home and help protect the environment.
But a West Yorkshire Police spokeswoman said: "We received a number of phone calls from alarmed customers who feared the contents were more sinister. In view of last year's anthrax attacks in America, the recipients feared a repeat phenomenon."
A spokesman for Yorkshire Water said: "The decision to send out these water conservation devices in plain envelopes without the Yorkshire Water logo on was a mistake. each one did, however, contain an explanatory letter printed on company stationery and a water conservation leaflet which also bears the Yorkshire Water brand."