The number of American workers and job applicants testing positive for use of illegal or abusable drugs fell 1990 for the fourth straight year, according to figures released last week by a major drug-testing company.

Of 1.9 million people tested in 1990 by SmithKline Beecham Clinical Laboratories, 11 percent tested positive, compared with 12.7 percent in 1989, 13.6 percent in 1988 and 18.1 percent in 1987.

Included in the 1990 figure were more than 240,000 people who either worked in or were applying for transportation jobs -- "safety-sensitive" positions with airlines, railroads, bus and trucking companies that under a year-old federal law require periodic drug testing. A total of 2.95 percent of those subjects tested positive, according to the King of Prussia, Pa.-based company, which does about 25 percent of such government-mandated tests.

Other findings from the report:

The proportion of those testing positive for drugs was highest in the West, at 12 percent, with the Northeast showing the sharpest decline -- from 21 percent in 1989 to 11.9 percent last year.

Marijuana continued to be the most common drug. Other drugs detected, in descending order of frequency, were cocaine, opiates, amphetamines and the hallucinogenic tranquilizer PCP.

A study published recently by Workplace Trends newsletter says the total number of companies announcing permanent staff cuts in 1990 was 224, up four-fold from the previous year.

In the fourth quarter alone, 72,205 job cuts were announced -- about 47 percent higher than the prior quarter.

The financial services industry led the trend by eliminating 21,681 positions. The automotive industry was next with 18,070 staff cuts.