The structure of the ribonucleoprotein (RNP) complex of 3 coronaviruses was investigated. A single-stranded helix of diameter 14-16 nm and up to 320 nm in length was released from disrupted particles of human coronavirus strain 229E and mouse hepatitis virus strain 3 after incubation in mild conditions. The helical complexes appeared to be composed of globular subunits with long axes of 5-7 nm surrounding a hollow core of diameter 3-4 nm. The complexes were sensitive to both pancreatic RNase and to Pronase. No undegraded internal component was obtained from disrupted avian infectious bronchitis virus particles. These structures are apparently RNP complexes. The similarity between these RNP and those of other large lipid containing RNA viruses is discussed.