Aims: The objective of this study was to access APC-effector cell cluster formation in genetically susceptible BALB/c (H-2d) mice infected with highly virulent Moscow strain of ectromelia virus (ECTV-MOS) and estimate of lymphocyte activation based upon expression of CD62L and CD44 molecules. Methods and Results: APC-effector cell clusters were obtained by enzymatic digestion from draining lymph nodes (DLNs) and spleens of BALB/c mice. We found that APCs infected with ECTV-MOS form unstable clusters with effector cells, and thus may diminish T-cell activation at the early stage of mousepox. Different types of effector cells including T-cell subsets (CD4+ and CD8+), B cells and polymorphonuclear cells colocalize within individual clusters. Increase in CD19+ B cells within APC-effector cell clusters during severe clinical mousepox may reflect B-cell activation. Conclusions: Our studies indicated vigorous changes in APC-effector cell cluster formation in genetically susceptible BALB/c mice during mousepox (up to 2 weeks). ECTV-MOS can modulate APC interactions with effector cells and consequently may impair T-cell activation probably owing to unstable cluster formation and/or subsequent weak stimulation by infected APCs at the early stages of mousepox. Significance and Impact of the Study: This is the first report of APC-effector cell cluster formation in BALB/c mice during mousepox. It gives us a new light on the mutual cell-cell interactions and development of the immune response during ECTV-MOS infection.