The expression of a Mycoplasma gallisepticum (MG) F-strain surface epitope was studied using monoclonal antibody 6F10. Different F strain colonies were cloned based on positive or negative expression of the 6F10-recognized epitope. Clones selected for the lack of expression of this epitope (negative clones) reverted rapidly to positive expression. On the other hand, negative clones derived from F-strain-vaccinated chickens expressed the 6F10-recognized epitope variably. After in vitro passages, two groups of cultures were observed. One group reverted rapidly to positive expression. The other group exhibited a very slow rate of reversion, with 30% or fewer of their colonies expressing the 6F10-recognized epitope. These results indicate that the 6F10-recognized surface epitope of F-strain is variably expressed under in vivo and in vitro conditions.