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Maize (Zea mays L.) haploid cells were cultivated in a 1500 ml aerated and stirred batch bioreactor using modified BM medium. Cell growth was highly affected by pH and dissolved oxygen, and we observed two fairly distinct growth phases. During the first two days after inoculation at pH 5.8, oxygen consumption was high and the cells lowered the pH to a value around 4.3. After this period the pH stabilized at 4.5 and the dissolved oxygen reached a steady level. Decreasing dissolved oxygen concentration leads to lower growth rate and to higher pH. Both events mean stress conditions for the cell culture and probably result in increased genetic variability, and the loss of regeneration capacity. The stress condition during the adaptation phase can be eliminated by decreasing the pH of the medium to 4.7 before inoculation and by keeping dissolved oxygen above 40%. These conditions provide prolonged exponential growth dynamics and the cell suspensions could be the basis of large scale cultures also.Abbreviations: 2,4-D-2,4-dichlorophenoxyacetitc acid; NAA-naphthalene acetic acid