[en] Several endpoints have been used for monitoring human population that have been exposed at work or in the environment to genotoxic agents, particularly to ionizing radiation. The cytokinesis-block micronucleus (MN) assay in peripheral lymphocytes is a reliable method for evaluating radiation induced chromosomal damage (DNA breaks and mitotic spindle disturbances) and thus, a suitable dosimeter for estimating in vivo whole body exposures. A research to determine the influence of age, sex and life style factors (smoking habits) on the MN spontaneous and radiation induced frequencies was carried out in order to define the use of this assay in Biological Dosimetry. The estimation of MN frequencies was analyzed in lymphocytes cultures from 50 health donors aged between 4 and 60 years. Based on the smoking habits, they were divided into 2 groups. A fraction of the sample was irradiated in vitro with γ-rays in the range of 0.35 Gy to 4 Gy. A statistically significant influence on the spontaneous MN frequency was observed (R² = 0.59) when the variables age and smoking habit were analyzed, and a statistically significant influence on the radiation induced MN frequency was also obtained (R² = 0.86) when dose, age and smoking habit were studied. Sex did not influence significantly MN variability, but there was a greater dispersion in the results obtained from female donors, when compared to males, possibly due to the loss of X chromosomes. The comparison of the data from smoking donors to the data from non smoking donors supports the convenience of taking into account the smoking habit for estimating in vivo whole body exposure to γ-rays for doses below 2 Gy. (authors). 8 refs., 3 figs., 2 tabs