United Airlines, the world's largest airline, yesterday reported a first-quarter 1986 net loss of $107.3 million compared with a net loss of $3.9 million for the same period a year ago.

Richard J. Ferris, chairman and chief executive officer of UAL, attributed the loss to a combination of greater airline operating losses, foreign-exchange losses and increased interest expense.

Also yesterday, Sears, Roebuck and Co., the nation's largest retailer, reported first-quarter net income of $195.3 million (52 cents a share), down 12.5 percent from net income of $223.3 million (60 cents) a year ago.

Sears' revenue rose 7.1 percent to $9.39 billion from $8.77 billion.

In the oil sector, Amoco Corp., the nation's fifth-largest oil concern, said its first-quarter profits sank 30 percent from a year ago because of plummeting oil prices.

Also, E. F.Hutton Group Inc. reported sharply higher profits for the first three months of the year, bolstered by surging stock and bond markets.

*United Airlines posted revenue of $1.48 billion, up from $1.43 billion in the first quarter of 1985.

Meanwhile, UAL Inc., United's parent, reported a first-quarter 1986 net loss of more than $103 million ($2.48 a share), compared with a $661,000 (19 cents a share) net loss in the first quarter of 1985.

UAL's revenue for the quarter was $1.96 billion, up 23 percent compared with $1.59 billion in the previous year.

*Sears, Roebuck and Co.'s merchandise group's first-quarter income fell to $42.1 million from $76.5 million a year ago, reflecting generally flat sales in a highly competitive retail environment, the company said in a prepared statement.

Income for Sears' Allstate insurance group was $176.4 million, compared with $144.3 million last year.

Dean Witter financial services group and Sears World Trade Inc. reported losses of $6.4 million and $3.2 million, respectively, while Coldwell Banker real estate group income dropped nearly 50 percent from $31.5 million to $16.8 million.

*Chicago-based Amoco said its net earnings for the three months ended March 31 came to $331 million ($1.28 a share), compared with $473 million ($1.76) in the same period a year ago.

Amoco's revenue fell 15.7 percent to $5.9 billion from $7 billion.

*E. F. Hutton said its after-tax income came to $40.1 million ($1.27 a share) on record quarterly revenue of $895 million during January-March.

In last year's first quarter, Hutton's profit totaled $24.1 million (90 cents) and revenue was $759 million.