A cosmid library has been constructed from a hamster x human hybrid cell line and gridded into 270 microtiter plates containing a total of 25,920 single colonies. Approximately 84% of the recombinants contain human material, with an average length of 29 kb. This library represents a nearly three-fold coverage of human chromosome 4. We investigated this library for presumptive genes, using a set of oligonucleotides detecting SpI and splice-site consensus sequences. The presence of simple repeat motifs was investigated in the cosmids using the oligonucleotides (GGATTT)3, (GGAT)4, (CAC)5, (GCC)5, (AGC)5, (GATA)4, (GACA)4, and (CA)8 as hybridization probes.