Cytoplasmic crystalline inclusions could be isolated and partially purified from influenza/WSN-infected chick fibroblasts by extraction of the cytoplasmic homogenate with fluorocarbon, followed by sedimentation to equilibrium in potassium tartrate gradients. Repeated analyses of the crystal-enriched fraction by PAGE showed it to be predominantly protein (mol. wt. 23,000 daltons) with a minor contamination of 2-10% by a protein of mol. wt. 60,000 daltons, which comigrated with the NP protein. The latter is believed to exist within characteristic cytoplasmic and nuclear helices, also described in this study. The molecular weight data suggest that the crystals are composed of the so-called nonstructural protein.