Screening of the occurrence of double minutes (DM) was performed in more than 1,000 metaphases obtained from a total of 22 solid human breast tumours and more than 3,600 metaphases from a total of 55 malignant effusions (45 patients with different types of carcinomas). DM were observed in 15 of these breast tumor cases and in 34 of the effusions (obtained from 29 cancer patients). The percentage of cells exhibiting DM as well as the number of DM per respective cell varied widely. It could be seen that metastatic cells from malignant effusions exhibited on the average more DM per cell than did cells of primary breast carcinomas. Differences in the incidence of DM could be observed between different carcinomas as well as between different age groups. In addition, it did not appear that DM could be induced by mutagenic tumor therapy. DM are thus not a rare finding in human solid tumors but, as cytogenetic equivalents of gene amplification, they rather represent a fundamental biological characteristic of tumor development.