March 15, 1983

International The first cut in OPEC oil prices in the organization's 23-year history was agreed upon by its 13 members in hopes of regaining control over the world petroleum market. The key cut is a 15 percent reduction - to $29 a barrel, from $34 - in the price of Saudi Light crude and production quotas for all OPEC members except Saudi Arabia. (Page A1, Column 6.) President Reagan stressed to Israel the ''urgency'' of its agreeing on terms for a withdrawal of all its forces from Lebanon, according to an Administration official. Mr. Reagan made the statement to Foreign Minister Yitzhak Shamir, who also met with Secretary of State George P. Shultz for five hours. (A1:3.)

An article about Lever Brothers in Business Day on Saturday incorrectly reported the ownership of a soap called Monchel. It is a Procter & Gamble product.

When critics say the Reagan Administration may have underestimated the cost of its long-range military spending plan, Defense Secretary Caspar W. Weinberger argues that the Pentagon has shed the bad habits that push up the cost of developing and buying weapons. Two years ago the newly installed Reagan team at the Defense Department approved a list of 32 measures to improve the costly and lengthy military procurement system. But there is disagreement and uncertainty about how well the department has succeeded in carrying out its ''acquisition improvement program.'' The argument is important in both practical and political terms. Some members of Congress want to see the long-range spending plan ''repriced.'' If a recalculation showed that future costs had been underestimated, the Administration would either have to scale back its ambitious plan to ''rearm America'' or ask Congress for even larger sums.

American Airlines, in a sweeping attempt to simplify fares, announced a plan yesterday to replace thousands of different fares with four basic ones tied to the distance traveled. Expressing the hope that the rest of the industry would follow suit, Thomas G. Plaskett, senior vice president for marketing, said: ''The new streamlined fare structure will help reduce fare confusion and make airline ticket prices more equitable for everyone.'' Two major carriers, Trans World and United, reacted favorably to the plan. But other airlines were doubtful that the new system would be supported in highly competitive markets.

BROWN chunk of rock, a meteorite found on the ice of Antarctica four years ago, has sent a shock wave of excitement through the laboratories of planetary science. Its drab appearance belies its apparently exotic provenance. The rock very likely comes from Mars. If scientists are right about this, and the evidence is becoming more and more persuasive, the meteorite would assume an importance in the history of science comparable to that of the first Moon rocks returned by the Apollo astronauts. It would be the first known object from another planet to reach the Earth. It would afford scientists their first chance to study in detail the chemistry and geology of Mars. Tests on pieces of the meteorite have recently erased most uncertainty about its Martian origin. After an analysis of gases trapped in the meteorite, Dr. Robert O. Pepin, a University of Minnesota physicist, emerged from his laboratory last week and exclaimed: ''It's from Mars. I don't think there's any doubt.''