A technique for DNA amount determination by flow cytometry based on the use of 7-amino-actinomycin D (7-amino-AMD), a fluorescent analogue of antibiotic actinomycin has been investigated, and a particular staining procedure has been developed. The procedure includes short fixation in 70% ethanol and staining for 20 min in 10W5M solution of 7-amino-AMD at pH7. The results of DNA content measurements are very reproducible. The histograms obtained have a coefficient of variation less than 3%.The absorption maximum of the complex of 7-amino-AMD with DNA is situated in the green spectrum region, making this stain particularly suitable for argon laser flow cytornetry.