First Security Corp. reported first-quarter net income of $8 million, an increase of 11.5 percent over first-quarter 1987, a jump of 33.5 percent over fourth-quarter 1987, and the bank holding company's highest quarterly earnings in four years.

First Security Chairman Spencer F. Eccles attributed the increase primarily to improved margins.Total loans and leases held by the corporation's subsidiaries rose from $3.3 billion in first-quarter '87 to $3.4 billion this year, said Eccles.

"Total assets, on the other hand, were $4.9 billion at quarter's end, down 3.3 percent from one year ago, reflecting our planned balance sheet adjustments which increased margins and improved our loan to deposit ratio."

As a result of this strategy, he said, average return on earning assets for the quarter increased from 9.09 percent to 10.06 percent. Even though average cost of funds rose slightly, Eccles said, the corporation's average margin climbed from 3.65 percent a year ago to 4.46 percent in first quarter '88.

Increased loan volumes also were a factor, said Eccles, as First Security subsidiaries made loans of $837.1 million in the first quarter compared to $777.7 million in the same three months last year.

"Our increased earnings are not only the result of higher loan volumes and improved margins," said Eccles, "but also of our continued success in controlling expenses and reducing non-performing assets."

He said problem credits are now at their lowest level in more than five years. "This planned liquidation of a considerable portion of our non-performing assets, along with moderate improvement in the economy, was a key factor in First Security's improved performance."

Non-performing assets were reduced to $109 million, or 3.1 percent of total loans, leases and other real estate owned. This was down from $159 million following first quarter '87 and $115 million at the start of 1988.