The general background to tumour analytic work using quantitative optical cytochemical methods is first presented. An instrument complex, constructed especially for multiparameter work in cytopathological material has been developed. Nuclear changes have been followed in cell populations during their development through different grades of atypia to cancer and conspicuous cytochemical changes were observed. In a comprehensive series of clinically verified mammary carcinomas, a large percentage of cases was found in which the DNA values were within the normal range, while the others showed pronounced aneuploidy. A clear correlation was found between DNA profile-type and patient survival, the latter of which reflects the degree of malignancy in the individual case. The shift from resting state (G0) to growth activated G1-stage is initiated by a large increase of the nuclear proteins. Mammary tumours of a high malignancy grade, as judged by their DNA profile type, showed an especially great accumulation of nuclear protein and thus a high degree of activation. DNA-profile measurements, preferably combined with determinations of nuclear proteins can thus be used for judging malignancy grades in mammary tumours, which is also of considerable clinical interest. An as yet limited observational material also indicates similar situations in some other types of tumour.