In an effort to further divest itself of stock in companies with holdings in South Africa, the Smithsonian Institution announced yesterday it will sell its holdings, worth at least $8.6 million, in an international equity portfolio.

The action, which was decided at Monday's Board of Regents meeting, was taken because the investment could be "perceived as inconsistent" with a Smithsonian policy barring investment in companies involved in South Africa that do not adhere to the Sullivan Principles.

"It was a question of the wisdom of holding this particular fund," said Smithsonian spokesman David Maxfield. "Especially in light of the other funds being disposed of in the year when the companies were not signatories of the Sullivan Principles."

The Smithsonian's executive board last June 17 instructed the museum to sell its holdings in those American companies without the nondiscriminatory hiring procedures that govern the principles, and also ordered its investment managers not to buy stocks in such companies.

Carlisle Humelsine, acting chairman of the regents' investment policy committee, determined that the Trustees Commingled Fund-International Equity Portfolio (part of the Vanguard Group of Funds) could be perceived as inconsistent with the June policy. Although the portfolio itself does not have and never has had operations in South Africa, the action was taken as a "matter of appearances," according to a spokesman.

The Smithsonian had invested $11.5 million in the mutual fund at the end of 1984. The investment was reduced to $8.6 million (with a market value of $10.6 million) when a portion of the investment was sold in 1985. The fund accounted for 6 percent of the market value of the institution's total endowment as of December 1985.