Our results demonstrate that the combination of plant extracts obtained from in vitro plant cultures containing valuable secondary metabolites, such as betulinic acid, with traditionally-used chemotherapeutics, such as doxorubicin, has a stronger cytotoxic effect on leukemia (NALM6) cells than betulinic acid alone; this combination acts by inducing apoptosis through cell cycle arrest in the subG0/G1 phase, altering the level of apoptotic-related gene expression (TP53, PUMA, NOXA and BAX) and reducing mitochondrial membrane potential. The gene discussed is TP53; the disease is leukemia.