In the face of allegations that he has improperly used a tax-exempt think tank, which he founded five years ago, to advance his presidential prospects, Rep. Jack Kemp (R-N.Y.) this week removed himself as chairman of the organization. His entire board also has resigned.

Kemp's spokesman, John Buckley, said yesterday the changes at the Fund for an American Renaissance do not constitute an admission of impropriety.

Nor, he said, do the changes come in response to a lengthy front-page article on the foundation in Sunday's Buffalo News, a major paper in the congressman's district, or in anticipation of a threatened lawsuit by Kemp's Democratic challenger for Congress, Buffalo City Councilman Jim Keane.

Rather, Buckley said, the resignations, along with this week's first-ever disclosure of individuals and corporations that have given a total of $505,603 in tax-deductible contributions to the five-year-old fund, had been long planned.

They were made because "it is our intention to have the fund be purer than the driven snow in the eyes of anyone who looks at it," he said.

Kemp is one of four likely 1988 presidential contenders to have set up a tax-exempt foundation to finance public policy forums, seminars, publications and trips.

Under Internal Revenue Service regulations, such foundations cannot "participate or intervene" in political campaigns on behalf of any candidate.

Keane said yesterday he will file his lawsuit as planned today in federal court in western New York. The suit calls for revocation of the group's tax exempt status and alleges:That the fund has worked jointly with the National Conservative Political Action Committee (NCPAC), a political group that specializes in targeting liberal candidates for defeat. That the fund's expenditures for foreign trips taken by Kemp and its underwriting of television documentaries narrated by Kemp have all, directly or indirectly, advanced his nascent campaign for the Republican presidential nomination. That the fund has over the years shared staff and exchanged direct-mail lists with Kemp's political action committee.

Don Eberly, executive director of the fund, yesterday denied all charges.

He said the fund has never shared staff with Kemp's PAC, the Campaign for Prosperity, although he acknowledged that individuals who have worked for one have later worked for the other. He said the two groups have never exchanged direct-mail lists. He said financing of the foreign trips was permissible under IRS regulations.

Eberly acknowledged that the fund and NCPAC pooled their resources, along with a number of other conservative groups, in 1982 to pay for an in-depth survey of public attitudes coordinated by longtime Republican political consultant Eddie Mahe. The fund paid $8,750 for the survey. This, too, he said, was permissible.

Eberly said board members of the fund who are stepping down are Charles Black, a political consultant, informal political adviser to Kemp and one of the founders of NCPAC; Jim Roberts, who formerly served as a consultant to both the fund and Kemp's PAC; Randall Teague, the fund's legal counsel, and Sharon Zaleska and David Hobby, both members of Kemp's congressional staff.

Asked if the political backgrounds of several of the fund's departing board members suggest an overlap with campaign politics, Buckley responded: "That would be denigrating the portion of their brains that is able to think about public policy."

The other likely presidential candidates who have founded public policy think tanks are Sen. Gary Hart (D-Colo.), Arizona Gov. Bruce Babbitt (D) and television evangelist Marion G. (Pat) Robertson.

Sen. Robert J. Dole (R-Kan.) chairs a tax-exempt foundation that seeks to find employment for disabled people.