Many human tumours are hyperdiploid, particularly in advanced stages of growth. The purpose of the present work was to investigate whether exposure to hypoxia followed by reoxygenation might induce hyperploidisation of diploid human tumour cells in vitro. The investigation was performed by using the diploid melanoma cell line BEX-c (median chromosome number, 46; DNA index, 1.10 +/- 0.04) as test line and the hyperdiploid melanoma cell line SAX-c (median chromosome number, 61; DNA index, 1.42 +/- 0.03) as control line. Cell cultures kept in glass dishes in air-tight steel chambers were exposed to hypoxia (O2 concentrations < 10 p.p.m. or < 100 p.p.m.) at 37 degrees C for 24 h. DNA content was measured by flow cytometry. Metaphase spreads banded with trypsin-Versene-Giemsa were examined to determine the number of chromosomes per cell. An electronic particle counter was used to measure cell volume. The expression of p53 and pRb was studied by Western blot analysis. Transient exposure to hypoxia was found to induce a doubling of the number of chromosomes in BEX-c but not in SAX-c. The fraction of the BEX-c metaphase spreads with 92 chromosomes was approximately 10% at 18 h after reoxygenation, decreased to approximately 2% at 7 days after reoxygenation and then increased gradually with time. The whole cell population became tetraploid within 25 weeks. BEX-c and SAX-c behaved differently during the 24 h hypoxia exposure. Cell volume and fraction of cells in G2 + M increased with time in BEX-c but remained essentially unchanged in SAX-c. On the other hand, the expression of p53 and pRb was similar for the two lines; hypoxia induced increased expression of p53 and hypophosphorylation of pRb.