Charles Z. Wick, director of the U.S. Information Agency, acknowledged yesterday that there was "no justification" for giving a $4,800 merit raise to Caspar Weinberger Jr., son of the defense secretary and one of a number of relatives of senior administration officials hired by the agency.

Wick also disclosed that his agency hired several other officials' relatives last summer as temporary clerical employes, including John Gergen, nephew of White House spokesman David Gergen; Julia James, daughter of former White House personnel chief E. Pendleton James; Melissa McFarlane, daughter of National Security Council staff member Robert C. McFarlane, and Susan DeLorenzo, a niece of McFarlane.

Another of McFarlane's daughters, Lauren, was hired in USIA's public liaison office for four months last fall, and James K. Malone, son of Assistant Secretary of State James L. Malone, was given a 12-month clerical job that expires this month. The temporary employes were paid at an annual rate of between $8,342 and $13,369.

Wick made the disclosures in a letter to Sen. Edward Zorinsky (D-Neb.), who had requested detailed information on the USIA's hiring and grant award practices and made the responses public yesterday. Zorinsky described the responses as "refreshingly candid" and said he now believes that Wick "recognizes USIA's serious management problems."

A USIA spokesman said that Wick had "responded fully and completely to all of Sen. Zorinsky's questions . . . . Director Wick has taken prompt action to fully investigate and promptly correct the problems to which the senator referred."

Wick said that while he initially approved the hiring of Weinberger's 36-year-old son he did not know the younger Weinberger had received a $4,800 merit increase until the issue was raised in press accounts. Weinberger earned about $50,000 a year as a USIA public liaison officer in New York until he resigned last month, citing excessive publicity over his job.

Wick said the White House personnel office had referred the names of Monica Clark, daughter of national security affairs adviser William P. Clark, and Anne Collins, a friend of the Clark family, for the permanent jobs they later received at USIA. Wick said he personally referred Barbara Haig, daughter of former secretary of state Alexander M. Haig Jr., for the interviews that led to her job.

A former USIA director recommended Daniel Wattenberg, the son of Ben J. Wattenberg, director of the Board for International Broadcasting, for another USIA post, Wick said.

On another issue, Wick said there was an "ethical conflict of interest" involving an agency grant that Zorinsky had questioned.

USIA made the $170,000 grant last year to the Mid-America Committee for International Business and Government, which in turn hired a consultant, Ian McKenzie, to advise Latin American and Arab nations on dealing with the U.S. press.

The USIA grant began in November as McKenzie's private contract to advise the government of Haiti on media relations was expiring, Wick said. He said McKenzie had received no payments from Haiti for several months and had told the Justice Department he was an "inactive" foreign agent for Haiti.

Wick said he has ordered "rigorous new guidelines" to enable his agency to determine whether future grant recipients have foreign contracts that could conflict with their USIA assignments.