The proportion of chloroplast DNA in total DNA from spinach leaves has been measured using the second order reassociation kinetics of a 3H-labelled chloroplast DNA probe in total DNA extracts. There was no significant difference between the proportion of chloroplast DNA in the basal and distal halves of 2 cm leaves and in the distal halves of 5, 8, and 10 cm leaves. The mean of all the observations was 21.1 ± 0.7%. There was little change in the average total DNA content of cells from any of the leaves but cells from larger leaves contained 130–170 chloroplasts while cells from the basal half of 2 cm leaves contained about 20 chloroplasts which were smaller than those from the larger leaves. Consequently the average number of copies of the plastome per chloroplast in large leaves was about 30 (5 × 10−15 g DNA) and in the smaller chloroplasts in the base of 2 cm leaves was 200 (32 × 10−15 g DNA). Staining with the DNA fluorochrome 4, 6-diamidino-2 phenyl indole (DAPI) showed 10–15 plastid nucleoid areas in chloroplasts of larger leaves, suggesting there are 2–3 copies of the plastome per plastid nucleoid.