Id family genes are widely expressed in hematopoietic cell lines. Expression of the four Id family genes was assayed by Northern blot in total cellular RNA isolated from 18 murine and human hematopoietic cell lines. Signal intensity was measured by phosphorimager before the blots were normalized by rehybridization to a 28S RNA oligonucleotide. Values are comparable only within the group of samples hybridized to a single probe; different exposure times (Id-1 exposed for 24 hours, Id-2 exposed for 120 hours, Id-3 exposed for 72 hours, and 28S RNA exposed for 30 minutes) were used for each probe to maximize clarity of the phosphorimage. Values were plotted as the relative intensity (pixel number) of the specific bands above background for each probe.