SummaryWe describe the incidence of 24 M-components among a population of 23,390, once or more examined blood-donors. They were followed up and controlled half-yearly. At the beginning of 1974 a medical check-up was performed with bone marrow smears, Rontgen examination, etc. We were able to distinguish 5 classes:1)myeloma or macroglobulinaemia2)probable myeloma or macroglobulinaemia3)essential monoclonal gammopathy (m.g.)4)transient m.g.5)dubious m.g.Our data are very similar to those of other population studies. The appearance of essential and transient m.g. is discussed.