This protocol can be used to label probes shorter than 200 bp to high specific activity.

Make a standard PCR reaction and purify the product from the gel.
Use 10 to 100 pg of this fragment as template and amplify in the presence of

10 pmol of each primer,
10 M dATP, dGTP, dTTP
1 x polymerase buffer
1 U Taq polymerase
50 Ci -P32-dCTP (3000 Ci/mmol)
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45 l.

Denature at 94oC for 3 minutes and then do 15 cycles of
30 seconds 94oC
30 seconds Tm
2 minutes 72oC

After this add cold dNTPs to a final concentration of 150 M and do 5 additional cycles.