A homogeneous group (same sex, age and environmental conditions) of 35 Italian cattle of the Podolian, Friesian, and Romagna breeds was investigated concerning the spontaneous incidence of sister chromatid exchange (SCE). The mean values of SCEs/cell were 7.9 +/- 3.4, 7.1 +/- 3.3, and 7.3 +/- 3.2 in the Podolian, the Friesian, and the Romagna breeds, respectively, with significant differences between the Podolian and the Friesian breeds. Simultaneous disclosure of SCEs and fluorescent G-bands in the lighter chromatid made possible the identification of chromosome 1, in addition to the biarmed X and Y chromosomes. The inter-chromosomal SCE-distribution revealed a nonrandom pattern due to significantly increased values of SCEs in chromosomes 1 and X, particularly in the Romagna breed.