Possibilities and limitations in the use of in vivo cytogenetic tests on somatic cells are discussed. Special attention is focused on the problems raised by the relative insensivity of the available procedures. It is emphasized that in planning screening tests the accumulated experience from studies with known chromosome breaking chemicals must be taken into account; theoretical considerations should not override the actual observations. In the Appendix, technical details of the micronucleus test are given. Furthermore, some thoughts are expressed concerning cytogenetic testing on germ cells and embryos.