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The aim of this study was to explore the dose- and time-dependent effects of hydrophilic peroxyl radical initiator 2,2'azobis(2amidinopropane)dihydrochloride (AAPH) on apoptosis, and on expression of Bcl-2 in L1210 leukaemic cells. We observed a progressive increase of intracellular concentration of oxygen free radicals (OFR), manifested by the rise of 6-carboxy-2', 7'-dichlorodihydrofluorescein diacetate, di(acetoxymethyl ester) oxidation, during 24 h of cells exposure to AAPH. Oxidative stress was associated with peroxidation of cellular lipids, which was demonstrated by the measurement of thiobarbituric acid-reactive substances and conjugated dienes. Analysis of cell viability by the use of trypan blue exclusion method revealed that AAPH reduced the ability of L1210 cells to multiply or survive. AAPH increased the number of leukaemic cells with typical features of apoptosis like condensation of chromatin, pyknosis and fragmentation of nucleus, followed by secondary necrosis. A characteristic internucleosomal DNA cleavage, visualized as a DNA ‘ladder’ consisting of fragments that are multiples of 180-200 bp was also observed. The intensity of apoptosis was dependent on AAPH concentration, time of cell exposure and the availability of growth factors and nutrients in extracellular environment (FCS concentration). The novel observation is the increase of Bcl-2 level in L1210 leukaemic cells surviving an oxidative stress. The level of Bcl-2 protein significantly rose with increasing AAPH concentration, and time of cell exposure to this oxidant. This phenomenon could be the result of: (1) negative selection of cells with the lowest expression of bcl-2, being more susceptible to oxidative stress and (2) increased synthesis and/or decreased degradation of Bcl-2 protein as an adaptation to continuous OFR loading. In contrast to growth-promoting medium (10% FCS/RPMI), the maintenance medium (2% FCS/RPMI) did not cover cell requirements for progressive Bcl-2 increase at the highest AAPH concentration (2 mM) applied in this study. Several observations indicate that the increased Bcl-2 level in surviving L1210 leukaemic cells exposed to oxidative stress is a symptom of their natural defence against cellular lipids peroxidation and apoptosis. [ABSTRACT FROM AUTHOR]