General Electric Co. said yesterday that its first-quarter earnings rose 5 percent to $359 million ($1.57 a share) from $342 million ($1.50) a year earlier as sales increased by 4 percent to $6.09 billion from $5.88 billion.

"GE's diversity contributed to its modest gain from the strong first quarter in 1980 despite a persistent weakness in many segments of the economy," Chairman John F. Welch Jr. said.

He said GE economists are cautiously optimistic that a gradual improvement in the economy will come about later in the year.

J.P. Morgan & Co. Inc. said its first-quarter profits increased to $79.4 million from $70.8 million in the comparable 1980 period.

The bank holding company, whose principal subsidiary is Morgan Guarantee Trust Co., the nation's fifth-largest bank, reported that income per share before securities losses rose to $1.93 from $1.73 in the first quarter a year ago.

Net interest income -- the difference between interest paid out on deposits and interest taken in from loans -- was $235 million, up 7.4 percent from $218.9 million.

Loans, primarily by Morgan's foreign offices, accounted for most of the increase in loan volume, President Lewis T. Preston told stockholders at their annual meeting.

Morgan reported that first-quarter assets averaged $51.6 billion compared with $44.1 billion, and its return on average total assets was 0.62 percent, or 62 cents on each $100 of total assets, compared with 0.65 percent.

Earnings of Caterpillar Tractor Co. in the first quarter slipped to $142.4 million ($1.64 a share) from $143.9 million ($1.66) a year earlier in spite of a rise in sales to $2.191 billion from $2.1 billion.

Although dollar sales were up because of price increases, physical sales volume was off 6 percent.

Celanese Corp. earned $2.25 a share in the first quarter, down from $2.39 a year earlier, Chairman John D. Macomber told the annual meeting yesterday.

The lower 1981 first quarter net was on 1 million additional outstanding shares.

Net income of the fiber and chemical combine was even with a year ago at $36 million. Sales were $911 million, up from $847 million a year ago.