A cytogenetic analysis of 14 primary testicular nonseminomatous germ cell tumors has been carried out after short term tissue culture. The modal chromosome numbers ranged from 53 to 113, in agreement with flow cytometric determination of the DNA content of the tumors. At least one copy of an i(12p) was present in 12 tumors. Two tumors, however, lacked that marker. Some chromosomes are apparently overrepresented, whereas others are underrepresented, although some differences between seminomas and nonseminomas were noticed.