The value of man-rodent hybrids for detecting creatine kinase BB (CKBB) was greatly increased by the use of an agar technique, in extended incubation (20 hours) with a concentrated solution of creatine as substrate (8 mg/ml instead of 1 mg/ml as usual). The selection of a man-rodent hybrid without discordant between chr. 14 and nucleoside phosphorylase (NP) (a marker of this chromosome) contributed efficiently to the detection of the positive correlation between CKBB and chr. 14/NP. Among 22 independent hybrids, the following were observed : 8 NP + CKBB+, 3 NP + CKBB(+), 11 NP-CKBB- and 6 chr. 14 + CKBB+, 2 chr. 14 + CKBB(+), 11 chr. 14 CKBB-, 2 chr. 14/CKBB+ and 1 chr. 14/CKBB(+). The presence of the chromosome was noted + or - according to its presence in more or less than 30% of the cells. These results show that the gene for CKBB is located on chr. 14 and that the presence of this single chr. 14 is sufficient for the expression of human CKBB in man-rodent hybrids. Discordant results reported by others (Povey et al., 1979) may be due to the weakness of the human CKBB detection in man-rodent hybrids or to the bad conservation of this enzyme.