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Increases in RNA, protein and cell size were determined cytophotometrically during the cell division cycle of Tetrahymena. For these parameters different patterns were found. RNA accumulates slowly during G1 period and faster during macronuclear S. This agrees with the changing uridine incorporation rate which is at least partly related to the varying macronuclear DNA amount. Increases in protein content and cell size occur mainly during G1and G2. This pattern was confirmed by determining the RNA : protein ratio in individual cells. It is minimal at the end of the G, period. These findings and evidence from the literature suggest that initiation of DNA replication is under negative control by the relative RNA content of the cell.

The combination of microphotometer systems with different types of computers allows various possibilities of automatisation in fluorescence photometry. This paper presents several examples of automated fluorometry as testing of preparation and staining, determination of fluorescence intensity with instantanous background correction, controlling a scanning system for detection of immunofluorescence on microplates, rapid microspectroscopy of fluorescence emission, quantification of fading phenomena, help for chromosome analysis and also facilities of data correction and statistical data evaluation.

Human peripheral lymphocytes metaphase chromosomes, prepared for flow analysis were used to evaluate the resolving power of a new microscope-based flow cytometer "Leitz MPV flow". We were able to resolve the human karyotype into about 15 peaks after simple ethidium bromide staining and excitation with mercury arc lamp. These results showed that a simple flow cytometer with a common relatively inexpensive source of fluorescence excitation light, can compete with more sophisticated laser-illuminated flow cytometers for flow cytogenetic studies.