During the course of a survey of fragile site expression in lymphocytes from twins one member of a dizygotic pair was found to be mosaic for trisomy 8. One hundred fifty metaphases from this individual were analyzed (100 treated with aphidicolin and 50 untreated); 43% were 46,XY and 57% 46,XY,+8. No differences were observed between the treated and control cultures in either the proportions of normal and trisomic metaphases or the overall or specific fragile site expression in the normal and trisomic cells.