The relationship between cell kinetics and cell harvest time on sister chromatid exchange (SCE) and NOR (marker chromosome) association was investigated in cultured Macaca fuscata lymphocytes. The average number of SCE’s in individuals ranged from 3.77 to 9.07 per cell. The highest SCE rate was founc in cultures harvested at 60 h. The data suggest a decrease in SCE frequency between 60 h and subsequent cell harvest at 72, 84, and 96 h. NOR association frequency was also affected by cell kinetics, as well as by eel harvest time. The highest association frequencies were at 48 h, when the number of first mitoses was greate