Previous reports from this laboratory have indicated that certain biological and physical characteristics of Venezuelan equine encephalitis VEE virus are influenced by the host. Data from thermal stability experiments with partially purified VEE virus suggest that virus propagated in L cell monolayers possesses greater thermal stability than similar suspensions of virus derived from chick fibroblast CF monolayers. The addition of 0.1 bovine serum albumin did not eliminate this stability difference. Lipid analyses of VEE virus propagated in these cell culture systems indicate that differences exist in the relative concentrations of the following phospholipid components phosphatidyl choline, phosphatidyl ethanolamine, and sphingomyelin. Correlation between the difference in viral stability and alterations in the lipid composition of the virus has not yet been demonstrated.