Abstract A comparison was made of the extent of incorporation of radioactive phosphorus into infectious and incomplete influenza virus particles. It was found that regardless of the ID 50 HA ratios of the progenies, the CPM HA ratios were of the same order; i.e., noninfectious hemagglutinins were as radioactive as infectious virus. Following treatment of labeled standard elementary bodies (EB) with ether, 15 to 20% of the isotope appeared in the ether phase (EE). On separating the aqueous phase (EEB) into HA and S antigen fractions, it was noted that most of the remaining P 32 was associated with the latter. Chemical fractionation of labeled standard EB suspensions by a modification of the Schmitt-Thannhauser method confirmed previous reports that the isotope was mainly in the phospholipid (hot alcohol soluble) and in the nucleic acid (hot TCA soluble) fractions. The EEB preparations revealed a relative increase of radioactivity in the cold TCA fraction and decreases in the alcohol and hot TCA extracts. Thus exposure to ether resulted in the breakdown of some viral constituents. Most of the residual phospholipids (not removed by ether treatment) were found to be associated with the HA particles, whereas the labeled components soluble in hot TCA were largely recovered in the S fractions. On chemical fractionation of labeled incomplete virus preparations, it was noted that the relative concentrations of the isotope in the hot TCA extracts decreased with the reduction of the ID 50 HA ratios of the progenies. Corresponding increases were noted in the cold TCA fractions. However, the ratios between the CPM values of the hot TCA extracts and the S antigen units present in the virus were found to be of the same order regardless of the composition of the progenies studied. Various difficulties were encountered in these studies, which are discussed together with the significance of the results and their relation to previous biological, chemical, and physical observations.