The stock market recorded a slight loss today in a quiet pre-holiday session.

The Dow Jones average of 30 industrials slipped 1.21 to 870.50, concluding the week with a net loss of 5.19 points.

That ended a string of five consecutive weekly gains for the average, during which it climbed almost 60 points.

New York Stock Exchange volume slowed to a 2 1/2-week low of 26.78 million shares, against 32.90 million Wednesday.

One notable exeception to the generally sluggish trend was Cyprus Mines, a producer of copper, lead and zinc, which jumped 9 3/8 to 36 7/8.

Standard Oil of Indiana agreed to acquire the company, whose stock traded as low as 16 5/8 two years ago, for $42 a share.

The Dow Jones industrial average had fallen 7.01 points Wednesday as International Business Machines, the No. 1 ho the no. 1 holding of vesting institutions, reported a 13.2 percent first-quarter earnings increase that came in well short of many analysts' advance estimates.

Delines outnumbered advances by slightly more than a 4-3 margin in the daily tally on the NYSE, and the exchange's composite index dropped .12 to 57.53.

Nationwide turnover in NYSE listed issues, including trades in those stocks on regional exchanges and in the over-the-counter market, came to 31.01 million shares.

Standard & Poor's index of 400 industrials lost .38 to 113.72 and S&P's 500-stock composite index was down .31 at 102.00.

Ford Motor, which raised its quarterly dividend from 90 cents to $1 a share, gained 1 to 44 5/8.

Halliburton fell 4 1/8 to 66 1/2. The compamy warned of "difficult" first-quarter earnings figures.

At the American Stock Exchange, the market value index slipped .10 to 180.32. The NASDAQ composite index for the over-the-counter market closed at 133.54, of .03.* conter market closed at 133.54, off .030.