Union Trust Bancorp, with assets of $1.7 billion, reported first-quarter earnings grew 11 percent to $4.2 million ($1.70 a share) from $3.8 million ($1.53) last year.

J. Stevenson Peck, chairman of the Baltimore holding company, attributed the gains to improved net interest income and reduced loan-loss provisions.

Another Maryland company, Pargas Inc. of Waldorf, said yesterday it earned $5.6 million ($1.36 a share) in the first quarter, compared with $4.4 million ($1.05) last year.

Revenues were $95.9 million, compared with $78.5 million. The current figure includes $796,145 that Pargas billed to a customer but is in dispute.

Although a mild winter and conservation reduced the volume of Pargas' liquefied petrolum gas operations, improved coal shipments helped the company balance sheet.

Planning Research Corp. of McLean said yesterday that third-quarter earnings increased 38 percent to $2.3 million (32 cents) from $1.7 million (25 cents) in 1982.

Revenues increased marginally to $80.8 million from $80.4 million last year.

For nine months, the computer services firm earned $6.6 million (95 cents), compared with $5.3 million (78 cents) in 1982, although revenue declined to $236.3 million from $239.3 million.

The stronger performance was aided by the national upswing in the real estate industry, increasing the demand for PRC's multiple listing service.

American Filtrona Corp. reported first-quarter earnings of $1.2 million (53 cents a share) compared with earnings of $1.1 million (46 cents) for the first quarter of 1982.

Sales were $25.4 million compared with sales of $22.3 million a year before.

The Richmond firm, which manufactures fiber products, including cigarette filters and air and fluid filters, said that the 14 percent increase in sales resulted mainly from the acquisition of Dollinger Corp. on July 2, 1982.

Other revenues were down 52 percent because of lower interest income, the company said.

Total revenue for the quarter was $25.7 million, compared with $23 million a year before.

The company also reported that income taxes as a percentage of pre-tax income were 44.7 percent for the latest quarter in 1983, down from 51.5 percent for the first quarter of 1982.

The change resulted from lower tax rates on foreign income and various tax credits, the company said.

Radiation Systems, a Sterling firm that makes antennas for satellite communications, the military and air traffic control, reported profits of $2.4 million (81 cents) for nine months, compared with $1.7 million (58 cents) last year, a gain of 43 percent. Sales jumped to $15 million from $8.8 million, a 71 percent increase.

For the third quarter, Radiation Systems earned $981,000 (33 cents), compared with $652,000 (22 cents), while revenues jumped 71 percent to $5.5 million from $3.2 million.

Systematics General Corp. said its first-quarter profits jumped to $277,000 (18 cents), compared with $40,000 (3 cents) last year. Sales slipped, though, to $3.7 million from $3.8 million.