MCI Communications Corp. yesterday reported second-quarter profits of $4.06 million (5 cents a share), nearly triple the telecommunications company's profit level for the same quarter last year.

This was the second successive quarter that MCI's profit topped $4 million. Profits were $1.39 million (1 cent) for the same period last year. Revenues also shot up dramatically, to $52.7 million from $33.4 million.

Much of the revenue and profits jumps is attributable to MCI's launching of residential long-distance telephone service this year. In fact, the number of MCI residential customers was more than 100,000, passing the number of business customers (about 7,000) for the first time during the quarter.

But MCI's profits rose by only $34,000 from their first-quarter level because of the cost of advertising the newly introduced residential service.

In another report, Communications Satellite Corp. said yesterday that the startup costs of its Satellite Business Bureau Systems venture resulted in a slight drop in third-quarter profits.

Comsat reported profits of $9.55 million ($1.19 a share) compared with $10.13 million ($1.27) for the same quarter last year and revenues of $75.52 million compared with $69.47 million.

Nine-month profits were $28.57 million ($3.57) compared with $32.64 million ($4.08) a year earlier, and revenues rose to $217.76 million from $193.17 million.