Variegation of transgene expression, a heterocellular or mosaic pattern of expression seen in all mice in a given transgenic line, is a frequently observed but unexplained phenomenon. We have encountered variegation with globin transgenes; when lacZ expression is driven by globin control elements a proportion of erythrocytes express ��-galactosidase (��-gal), while the remaining erythrocytes express none. The percentage of expressing cells is constant within each line (at any particular developmental stage), but varies between lines. Such variation may account for much of the line-to-line variability which has been reported in the expression of a transgene construct. We have now extended these observations by studying expression of several globin/lacZ transgenes with increasing age. Expression of ��-gal is variegated in all lines in adult mice, including those made with a ��-globin promoter and locus control region driving lacZ. The extent of variegation differs widely between lines, but in all lines there is a marked decline in the number of erythrocytes expressing ��-gal with increasing age. Progression of silencing continues long past the point at which globin switching is complete, suggesting that it is not related to this process. We observe that age-dependent silencing is most severe in high copy number animals, Increasing variegation of transgene expression with ageing of mice is likely to complicate interpretation of the developmental regulation of transgenes. We speculate that it reflects a general mechanism of epigenetic regulation.