A method of measuring the toner concentration of a developer containing toner circulating in an electrophotographic reproduction machine including preparing a sample from the developer for measurement, arranging the sample so that the toner can be estimated as a proportion of the required toner concentration of the developer, irradiating the sample by Beta rays that are effected by the toner and measuring the Beta -rays being scattered back from the irradiation. An apparatus for measuring the toner concentration of a developer including an endless belt mounted for movement with part of its surface horizontal for forming a uniform layer of developer thereon for measurement, a measuring head as a source of Beta -rays positioned to irradiate the uniform layer of developer, a detector for measuring the effect of the irradiation, an electronic unit responsive to the measured radiation and a toner supply device operable by the electronic unit to replenish toner in the developer when needed. In lieu of the endless belt the photoconductive surface of the electrophotographic reproduction machine may be provided with one or more electrodes to which an electrical potential may be applied.