Intravascularly injected horseradish peroxidase selectively labels certain classes of cells in the brains of chick embryos: phagocytes, which have characteristic distributions and resemble either gitter cells or microglia; and some, but not all, dying neurons. Healthy neurons are not labelled. If the isthmo-optic nucleus is caused to degenerate by an intraocular injection of colchicine on the opposite side, most of its neurons take up peroxidase. However, destroying the afferents to the isthmo-optic nucleus increases its loss of neurons without affecting the number labelled. In sections double-reacted for horseradish peroxidase and endogenous acid phosphatase, all, and indeed only, the peroxidase-labelled cells exhibit intense, clumped acid phosphatase activity which resists glutaraldehyde fixation. This is true of all cell types in both normal and operated embryos. Even healthy neurons exhibit acid phosphatase activity, but this can be distinguished, because it is largely inhibited by fixation with glutaraldehyde.