1. An indirect measure of the numbers of stem cells in bone marrow used to reconstitute the immune system of mice that have been irradiated. The grafted cells form colonies in the spleen (CFU-S), each colony a clonal derivative of a single stem cell. CFU-E are the derivatives of a partially committed stem cell of the erythoid lineage.
2. A measure of the number of live bacteria or fungi in a (diluted) sample which is put into appropriate culture conditions, e.g. on a nutrient-agar plate; each colony represents a single organism in the sample. A similar approach can be used with viruses by plating them on to a culture of host cells.