Structural specificity of binding and cytolysis of HCMV-infected human foreskin fibroblasts (HFF) by human NK and LAK cells was studied in inhibition assays. A sample of 60%-deacetylated alpha-D mannose penta-acetate was used as inhibitor that was previously shown to specifically inhibit binding and cytolysis of tumour target cells by human NK and LAK cells. We found now that cytolysis of HCMV-infected HFF was inhibited in a dose-dependent manner showing complete inhibition at concentrations above 640 nmoles/ml mannose acetate. This effect on cytolysis was based on inhibition of conjugate formation between virus-infected cells and CD56+NK and LAK cells. In the presence of mannose acetate (640 nmoles/ml) conjugate formation of virus-infected cells was suppressed down to the level of uninfected cells. The latter showed residual conjugate formation on the basis of adhesive interactions with chemospecifity other than for mannose acetate, which were not capable of triggering cytolytic reactions. Coculturing of target cells with LAK cells appeared to induce expression of additional mannose acetate-specific target sites yielding increases of conjugate formation and cytolysis.