When different antigenic variant clones are injected in equal numbers into white mice one variant clone always replaces the other. This phenomenon appears to be a predictable one, even under conditions analogous to a chronic infection. It is hypothesized that a constant ratio is approached between the number of cells of different antigenic serotypes present in a single population, in such a manner that there is always a major antigenic variant and minor populations of different antigenic variants. It is further suggested that these ratios can undergo rapid changes in response to changes in the environment, e.g. nutritional status of the host, changes in body temperature, antibody synthesis, etc. The changes in these ratios are discussed in relation to the mechanism(s) of antigenic variation.