Analysis of DNA content in cultured cells derived from 5 benign and 8 malignant human brain tumors was performed by flow cytometry, using propidium iodide as fluorochrome. Normal, non-stimulated human lymphocytes were used as controls. Cells harvested from the first confluent subculture had DNA distribution histograms similar to those of a replicating, non-synchronous population of diploid cells. This observation was made on all cultures, regardless of the DNA distribution patterns in the original tumors, including two metastatic tumors which originally had a predominantly triploid DNA content. After 3 to 5 transfers an increase in the proportion of cells with a wide range of DNA content was observed in some cultures. These changes were usually associated with a loss of cell viability. These observations suggest that in the experimental setting described, in vitro proliferation of human tumor cells may be selectively associated with cells with diploid amounts of DNA.