The main structural nucleocapsid protein, NP, of parental influenza virus (WSN, HON1) was modified in the infected cells early after inoculation. The modification took place within ribonucleoprotein particles in the cytoplasm but was not observed within ribonucleoprotein particles in the nuclei. It occurred when the cells were incubated at 37 degrees C and not at 4 degrees C. The modified protein migrated slightly faster in SDS-containing polyacrylamide gels. Peptide mapping of NP protein labelled with 125I- of 14C-amino acids showed the modified form of NP not to contain at least two peptides but to contain one additional peptide as compared with the unmodified precursor. This suggested that the modification could be due both to proteolytic cleavage and to covalent modification of an amino acid residue. According to the latter suggestion, the intense phosphorylation of parental NP was observed in the cytoplasma of the infected cells when 32P was added for 30 min one hour postinfection. The nuclei of the infected cells early after infection contained more than half of parental ribonucleoprotein particles in which, however, unmodified NP was present. The possible significance of the observed modification of parental NP in the infectious cycle of influenza virus is discussed.