33P is preferable to 35S for labeling probes used in in situ hybridization.

We compared RNA probes labeled with 35S-UTP and 33P-UTP for use in in situ hybridizations. 33P-UTP was readily incorporated into in vitro transcribed RNA, producing 33P-labeled riboprobes of high specific activity. When the 33P- and 35S-labeled riboprobes were compared in in situ hybridizations using two different tissues, we found that the 33P-labeled riboprobes were less "sticky" than the 35S-labeled riboprobes, giving significantly less nonspecific background hybridization. Because of the low level of background stickiness, it was possible to use ten times more 33P-labeled riboprobe than 35S-labeled riboprobe without appreciably increasing background hybridization. Our findings indicate that, in most cases, 33P is the isotope of choice when labeling probes for in situ hybridizations.