The stability of rotavirus infectivity during CsCl gradient purification and subsequent storage was examined using our standard SA11 wild type (SA11-Cl3), the SA11 4F variant (SA11-4F), bovine rotavirus B223, and a panel of bi- and triparental reassortants derived from these parental viruses. Viral stability was determined by the recovery of infectivity at each step during a standard CsCl purification protocol. SA11-4F was the most stable parent (91-93% recovery), SA11-Cl3 had intermediate stability (10-21% recovery), and B223 was least stable (0.5-7% recovery). Among the reassortants, the recovery varied from 0.5 to 88.6% of the initial infectivity and was determined primarily by the parental origin of genome segment 4. The greatest loss of infectivity occurred during Freon extraction, with smaller losses during the CsCl gradient, and the smallest loss during the virus pelleting step. Comparison of the stability of viruses grown in the presence or absence of exogenous trypsin revealed that, in general, viruses grown in the absence of trypsin were more stable during purification. During 4-5 months storage at 4 degrees, the differences in stability of parental and reassortant viruses were not as dramatic as during purification and were not significantly affected by the presence or absence of trypsin during growth. However, survival during storage was as low as 4% and as high as 100% and was also primarily dependent on the parental origin of genome segment 4. It was noted that bovine rotavirus B223 had higher density in CsCl than either SA11-Cl3 or SA11-4F. The observation of heterogeneity in density was investigated using reassortants. These results indicated that all reassortants had intermediate density and suggested that physical interactions among the structural proteins were responsible for the heterogeneity in density. The possible roles of viral structural proteins in rotavirus stability and the relationship between the stability and the density are discussed.