Unemployment in Maryland dropped to 4.6 percent in 1985, the third-lowest rate in the country, despite a slight decline in the number of expected new jobs and in the amount of capital investment last year, according to the Maryland Department of Economic and Community Development.

The unemployment rate, down from 5.4 percent in the previous year, was below that of Pennsylvania (8 percent), Virginia (5.6 percent) and Delaware (5.3 percent), the agency said.

Although the number of new jobs expected as the result of increased investment in the state dipped to 36,133 from 37,175, total employment in the state grew to 2.2 million, an increase of 28,000, or 1.3 percent, DECD reported.

"Maryland's economy remains healthy and dynamic, and the near-record capital investment in Maryland last year clearly demonstrates a strong confidence by the business community in Maryland's future," DECD Secretary Thomas H. Maddux said.

Capital investment of $1.5 billion in 1985 topped the $1 billion mark for the third consecutive year, but was down slightly from $1.6 billion the previous year, according to a survey by the DECD.

DECD said that 510 firms announced plans for new or expanded business in the state, down from 871 in 1984 and 607 in 1983. In the five suburban Washington counties, 170 companies will invest $337 million in new or expanded facilities. That investment is expected to bring 12,175 new jobs.

Martin Marietta Corp., the diversified aerospace conglomerate based in Bethesda, last week announced it has expanded its construction materials business with the acquisition of Weaver Construction Co.

Weaver, Iowa's largest producer of construction aggregates (crushed stone, sand and gravel), had sales last year of $25 million, compared with $200 million for Martin Marietta Aggregates in 1985. The Martin Marietta operation is one of the principal U.S. producers of construction aggregates, and operates from more than 100 locations in the Midwest and the Southeast.

Celanese Corp., a large petrochemical and plastics firm based in New York, has acquired a 3.7 percent interest in Nova Pharmaceutical Corp., with an option to increase its holdings in the Baltimore biotechnology firm by an additional 12.2 percent, States News Service reported last week.

In an agreement on March 21, Celanese bought 751,174 common shares for $13.31 per share for a total of $10 million. The following week, Celanese purchased a 10-year option to buy an additional 2.5 million shares for $15.81 per share, or about $39.5 million, States said.

Robert L. Mitchell, vice chairman of Celanese, has been a member of Nova's board of directors since March 1985.

In addition, Celanese is considering forming a joint venture with Nova to develop a commercial product using biodegradable polymers, a licensed product used in delivering anticancer agents and other drugs to the brain.

Capital Systems Group Inc., a Rockville computer consulting firm, has won a $5.6 million contract to provide computer research and support services for the environmental epidemiology branch of the National Cancer Institute. The contract is for four years.

Atlantic Research Corp., the Alexandria rocket maker, has won a $14 million award from LTV Aerospace and Defense Co. to design and produce the rocket motor for the Army Tactical Missile System. ARC said it expects more than $50 million in revenue over the life of the program.