Rates of antigenic variation were measured in vivo in several populations of cloned lines of Trypanosoma brucei before and after cyclical transmission through tsetse flies. Two cloned lines were adapted for use in laboratory conditions by extensive syringe passaging and rates of antigenic switching/cell/generation were less than 3 x 10(-6) and 1 x 10(-4) in each line. Rates of switching were then determined after fly transmission of the first line and generated per capita rate values of greater than 2 x 10(-3) in three of four populations examined. In the fourth population the switch rate was lower: less than 7 x 10(-5) switches/ cell/generation. These data show that rates of antigenic variation are several orders of magnitude lower in syringe-passaged lines, such as those routinely used in the majority of laboratory studies, compared with most recently fly-transmitted lines. They also show that the reduction in switching rate associated with syringe passaging is reversible and is thus unlikely to be caused by mutation.