When a high NaCl concentration was used to decrease selectively the synthesis of cell proteins, the synthesis of most cellular polypeptides was greatly diminished relative to human rotavirus proteins. Thus, in the presence of 150 mM excess NaCl, 11 viral polypeptides were clearly identified. However, hypertonic conditions also reduced viral protein synthesis to a different extent with individual proteins. No significant changes in viral protein synthesis occurred during incubation under the hypertonic condition for up to 6 h, and infectious virus yields of MA104 cells incubated in the hypertonic medium did not differ from the yields of untreated MA104 cells. These results indicate that hypertonic conditions provide a useful tool for qualitative studies of viral protein synthesis in human rotavirus infected cells.