COS-7 cells are a widely used fibroblast-like cell line derived from African green monkey kidney cells. They are often employed for the production of recombinant proteins and the study of protein function, gene expression, and viral replication. Here is a general protocol for culturing COS-7 cells:

1. Cell culture medium: Prepare Dulbecco’s Modified Eagle Medium (DMEM) supplemented with 10% fetal bovine serum (FBS) and 1% penicillin-streptomycin (P/S). Warm the medium to 37°C before use.
2. Thawing cells: Remove a vial of frozen COS-7 cells from liquid nitrogen storage and thaw quickly in a 37°C water bath. Gently swirl the vial to ensure even thawing. Once thawed, transfer the cells to a sterile 15 mL conical tube and dilute with pre-warmed culture medium. Centrifuge the cells at 300 x g for 5 minutes to pellet them.
3. Resuspending cells: After centrifugation, aspirate the supernatant and gently resuspend the cell pellet in pre-warmed culture medium. Count the cells using a hemocytometer or an automated cell counter and calculate the required volume of cell suspension for seeding.
4. Seeding cells: Seed the cells in a tissue culture flask or multi-well plate at an appropriate density (e.g., 1 x 10^4 cells/cm^2) to achieve 70-80% confluence within 2-3 days. Add the appropriate volume of culture medium to cover the cells, ensuring that they are evenly distributed.
5. Incubation: Place the flask or plate in a humidified incubator at 37°C with 5% CO2. Check the cells daily for confluence, morphology, and any signs of contamination.
6. Passaging cells: When the cells reach 80-90% confluence, they should be passaged to maintain their health and prevent overgrowth. Aspirate the culture medium and wash the cells with phosphate-buffered saline (PBS) to remove residual medium and serum. Add a sufficient volume of trypsin-EDTA solution (0.25% trypsin, 1 mM EDTA) to cover the cells and incubate at 37°C for 2-5 minutes to detach the cells. Monitor the detachment under a microscope, and stop the reaction by adding an equal volume of culture medium containing FBS. Gently pipette the cell suspension to break up cell clumps and transfer it to a conical tube.
7. Subculturing: Centrifuge the cell suspension at 300 x g for 5 minutes, aspirate the supernatant, and resuspend the pellet in fresh pre-warmed culture medium. Count the cells, calculate the required volume of cell suspension for seeding, and seed the cells in new tissue culture flasks or plates as described above.

Please note that this is a general protocol, and it is essential to consult the specific recommendations provided by the cell line supplier or any relevant literature for optimal growth conditions and handling procedures.