Allegheny Airlines yesterday reported the most profitable month in its history - net earnings of $7.9 million for May compared with $15 million in the same month last year.

The Washington-based company - sixth largest of the nation's domestic airlines - said revenues soared 23 percent during the the month to $51 million. Discounted air fares have contributed to the growing business of Allegheny and other airlines, industry spokesmen said.

Coupled with earnings in earlier months, Allegheny's May performance boosted profits for the first five months of 1978 to $5 million compared with a loss of more than $7 million a year earlier. Revenues for the five months jumped 16.5 percent from a year ago to $215 million.

Allegheny officials said May also set a record for passenger boardings (1.15 million in scheduled service).[TEXT OMITTED FROM SOURCE] with traffic during the first three weeks of June and heavy advance bookings showing a strong summer travel season.

Also contributing to Allegheny's profits have been flight equipment sales, which brought gains of $2.8 million in May and $5 million for the first five months, before taxes.

Schwartz Brothers Inc., a Washington-based retailer and wholesaler of phonograph records, reported an increase in profits for the first quarter ended April 30 to $39,618 (5 cents a share) from $3,045 (4 cents). Sales rose to $6.5 million from $6.2 million.

President James Schwartz said a decline in wholesale volume was more than offset by a "significant" gain in retail sales at the firm's 17 Harmony Hut stores. Two additional retail units will be opened this year, he added.

Schwartz recently paid its first cash dividend - 10 cents a share to owners of record May 10.

Neotec Corp., of Silver Spring, has reported higher profits for the year ended Jan. 31 and the first quarter of its new fiscal year.

The company, which manufactures electro-optical equipment for use in agricultural and industrial quality control operations, earned $677,872 ($1.51 a share in the recent year compared with $516,016 ($1.16) as sales increased to $5.2 million from $5 million.

In the first quarter of the new year, Neotec earned $106,534 compared with $71,951, an increase of 48 percent. Revenues dipped slightly to $1.2 million from $1.3 million.

President Robert Rosenthal attributed the gains last year to an Agriculture Department contract for 95 grain quality measurement devices (worth $1.16 million and the sale of a ground meat analyzer program to Foodtec Inc. in January, which resulted in a $386,417 profit.