Expression of H-2Kk and H-2Dk molecules was studied by indirect immunofluorescence on vaccinia virus-infected Ls cells (derived from the L929 cell line), using D-23b and ASA-21 monospecific alloantisera, respectively directed against the H-2.23 and H-2.32 private specificities of the H-2k haplotype. Our study demonstrates that the increase in vaccinia virus-induced antigens on the membrane of infected cells (as a function of the multiplicity of infection) is associated with a concomitant decrease in the expression of both H-2Kk and H-2Dk serologically defined private specificities. Absorption experiments of D-23b and ASA-21 sera, using infected or uninfected Ls cells, also indicate that after virus infection, H-2Kk and H-2Dk private specificities are equally altered in their serological expression. We finally show that this alteration consists in a mere quantitative decrease of H-2 molecules, since the cytolytic capacity of a rabbit anti-H-2 serum, specifically reacting against the constant part of the heavy chains of H-2 molecules, was significantly more absorbed by uninfected cells than by infected cells. We conclude that no preferential decrease in the expression of H-2 Kk and H-2 Dk molecules is induced during cell infection by vaccinia virus.