The H2 complex has an important role in determining susceptibility to viral and chemical leukemogenesis in inbred mice. This also applies to transplantable leukemias, within the syngeneic system. In this respect H2K is sensitive, H2d is relatively sensitive, and H2b is absolutely resistant to leukemia induction and transplantation. In our present study we investigated the effect of Cyclophosphamide, (a known chemical leukemogen) on onco/suppressor gene expression in CBA/Ca mice, very shortly after treatment with chemical carcinogen without any manifestation of tumour/leukemia symptoms. Here we describe, in a "short-term" experiment, the gene expression of Ha-ras, c-myc and p53 which was similar to the leukemia induction in a "long-term" experiment. H2K showed marked elevation in terms of onco/suppressor gene expression. H2b expression was modest and H2d turned out to be more or less silent. The results obtained from a short term gene expression investigation shows similarity to those obtained earlier from long term leukemia inducing experiments.