1. Dry NZY plates for 30-60 minutes at 37 °C or age at room temperature for a couple of days.

2. Melt NZY top agarose (not agar) and place in 45-50 °C water bath. 3 ml top agarose is needed for one 100 mm plate.

3. Make a serial dilution (1:100) of the phage stock in 10 mM Tris-HCl pH7.5, 10 mM MgCl₂, 50 mM NaCl.

4. Mix 100 µl of infection-ready host cells to 100 µl of diluted phage for one 100 mm plate.

5. Incubate at 37 °C for 15-20 minutes.

6. (optional) For libraries with blue/white selection, add X-Gal and IPTG to the top agarose.

7. Add 3 ml NZY top agarose (45-50 °C) to each 200 µl of infected cells.

8. Mix and quickly pour onto the center of a 100 mm NZY plate. Use 6.25 ml of NZY top agarose per 150 mm plate.

9. Gently swirl to ensure even distribution.

10. Allow NZY top agarose to set .

11. Invert plates and incubate overnight at 37 °C.

12. Count the number of plaques.