Many of the 60,000 federal retirees here, who may have thrown away an important notice from the government, will find their May annuity checks smaller unless they retrieve that letter from the trash or get a replacement fast.

Beginning in April the government will start withholding federal tax from checks of retirees--unless those retirees advise Uncle Sam via a special form that they wish to continue paying quarterly.

Letters advising retirees of the withholding started arriving here last week. The letters, from the Westinghouse Information Service and Office of Personnel Management, contained a form retirees must complete and return by Feb. 9.

That form gives them the option of staying outside of the withholding program or, in the case of those who are already having tax withheld, of keeping that arrangement.

Most retirees, who do not now have any withholding, must respond by Feb. 9. Otherwise they will be subject to automatic withholding, at the rate of a married person with three "withholding items."

Last Thursday this column advised retirees that the letters were coming and warned that they must respond by Feb. 9. Since then a lot of people have called or written to say that they threw the letters away thinking that they were junk mail. They say the Westinghouse Information Service letterhead and the return address (PO. Box 30 Iowa City, Iowa) confused them.

Unfortunately, it was not junk mail!

OPM hired Westinghouse (at 26 cents per item) to handle the replies from retirees, which an optical scanner is to read.

If you didn't get the letter or if you tossed it away, get a replacement immediately!

The OPM says you should send in your full name, address and civil service retirement number. They will send you a new form.

Send your request for a replacement to:

OPM, Annuity Services Division,

Allottment Center,

P.O. Box 961, Washington D.C. 20044.

The unit's telephone number is 632-7606, but it is very busy these days taking calls from retirees.