On Sept. 27, The Post published an article titled "They're Just Not Serious About Cutting Costs," referring specifically to the secretary of defense and myself. Such an accusation is patently wrong. The facts are that Secretary Caspar Weinberger and the Department of Defense under his leadership not only have been serious about cutting costs but have actually done it. Readers should be familiar with the following:

In 1980, the average annual cost growth in major weapons systems hit 14 percent. In 1981, Secretary Weinberger made reducing this figure his top management priority. He has been extraordinarily successful in his efforts. The Congressional Budget Office reported to Congress as early as July 1984 that cost growth had been dropping in the pres-ent administration. That was an understatement. It has been substantially eliminated. For the last four years (1983-86), cost growth has been less than 1 percent on an annual basis. In 1985 the cost of our major weapons systems as measured by CBO actually went down.

Measured in dollars of constant purchasing power, the unit cost of many major systems has declined in recent years. For example, flyaway unit cost (i.e., excluding such items as support equipment and spares) declined 41 percent between fiscal year 1983 and fiscal year 1987 for the AH-64 Apache helicopter, 33 percent for the UH-60A Blackhawk helicopter, 22 percent for the F/A-18 strike fighter and 47 percent for the AV-8B. Similar figures are 4 percent for the CG-47 Aegis cruiser and 14 percent for the Trident submarine. For the Phoenix missile the analogous figure is 33 percent; for the Air Force purchase of the HARM missile, 58 percent; and for the Stinger missile, 51 percent. There are many other similar cases.

We have dramatically expanded competition in defense contracting. If ''competition'' is defined as two or more bids/offers, just over 41 percent of the Defense Department's procurement dollars were ''competed'' in fiscal year 1982. By fiscal year 1986 that figure exceeded 57 percent -- and almost 22 percent more of procurement funds were in follow-on awards to contracts that were competed in earlier years.

Sixty-five programs have been submitted to Congress between fiscal year 1982 and 1987 for multiyear procurement funding, which is widely viewed as one of the most important ways to promote program stability and reduce costs. Of these 65 candidates, 45 have been approved by Congress, with estimated savings of $7 billion. In the current two-year president's budget request, we forwarded nine more multiyear procurement candidates, with estimated savings of $1 billion. (I regret to report that the House Armed Services Committee proposes to deny three of the six fiscal year 1988 candidates, and the Senate Armed Services Committee one; neither committee has acted on the three fiscal year 1989 candidates.)

Auditors are a key ingredient in controlling costs. In the Defense Contract Audit Agency -- by no means our only audit resource -- staff years authorized have grown almost 70 percent since fiscal year 1980, and dollar savings have more than doubled. The savings from DCAA audits alone are now running over $7 billion per year, and are one of the reasons that we have been able to sustain so much of the president's defense program despite deep congressional cuts in overall funding.

We have given the auditors stricter instructions than any administration in recent memory. Many costs that were reimbursed in the past are now restricted or flatly disallowed. Legislative lobbying costs, for example, are no longer reimbursed under government contracts. Costs associated with defense of fraud have been made unallowable in cases that result in conviction, judgment against the contractor, debarment or suspension, or that are resolved by consent or compromise. Regulations have been clarified to eliminate ambiguities or loopholes concerning allowable costs for public relations and advertising, compensation, entertainment, marketing, special tooling and travel. (For travel costs, contractors are now restricted to the same General Services Administration rates that apply to government personnel.)

To end the ''catch us if you can'' mentality that previously infected overhead cost audits, in March 1985 the secretary of defense began requiring senior officers of defense contractors to certify, under penalty of perjury, that their indirect cost submissions do not include costs unallowed by regulation or by the terms of the contract.

These are a few of the things we have done in the Department of Defense over the past seven years to cut our costs. They have led to significant savings to the taxpayer and with other initiatives will continue to benefit us in the future. They are the actions of people who are serious about cutting costs.

WILLIAM H. TAFT IV Deputy Secretary of Defense Washington