Sub-micron Al fibers have been tested mechanically in situ in a transmission electron microscope. Their plastic behavior appears to be strongly sensitive to their initial dislocation content. We report here observations on fibers that were obtained by a chemical etching of a eutectic alloy where isolated and stable sources operate. The strength of sources in fibers of different size has been directly measured. We show that the increase of the yield stress with decreasing fiber size can be directly attributed to the increase of the source strength.