DNA from a panel of inbred strains of mice and colony bred mice, isolated from different geographical locations, was hybridized to mouse V preB and lambda 5 probes under stringent conditions, indicating sequence similarities greater than 80%. The probe for lambda 5 detects one gene and the probe for V preB detects two genes (V preB1 and V preB2) in the inbred strains of mice examined under the stringency used. No restriction endonuclease fragment length polymorphisms (RFLP) were detected with the V preB and lambda 5 DNA probes among the inbred strains of mice using Bam HI and Hind III. Very few RFLP were detected among Mus musculus subspecies, and the intensity of the hybridization did not differ significantly with either DNA probe. The number of RFLP increased slightly when different species and subgenera were examined, and the intensity of the hybridization signal began to decrease in samples from the different subgenera, suggesting a slight decrease in sequence similarity for both V preB genes with increased time of divergence. Fewer RFLP were detected with the lambda 5 DNA probe. DNA from 11 different Mus species representing 4 subgenera, genetically isolated from laboratory mice for approximately 1-12 million years, continued to hybridize under high stringency conditions using both DNA probes. A comigrating lambda 5 and V preB restriction endonuclease fragment was detected in most of the samples examined, suggesting the close physical linkage of V preB1 and lambda 5 is maintained within the genus Mus. These results suggest that V preB1, V preB2 and lambda 5 have been present for over 12 million years.