Yes, horse serum can appear turbid after thawing, and this turbidity can be due to several reasons, including:

1. **Precipitation of Proteins**: Horse serum is rich in proteins, and upon thawing, some proteins may precipitate due to changes in temperature or freezing processes. This is often a normal occurrence and doesn't necessarily indicate contamination.

2. **Lipids**: Horse serum contains lipids that can aggregate and cause turbidity, especially if the serum was not properly mixed after thawing. Lipid separation can occur during storage and may lead to a milky appearance.

3. **Contamination**: While turbidity can be a normal phenomenon, it can also be a sign of contamination, such as bacterial or fungal growth. If the serum was not handled or stored properly, it could have become contaminated. Signs of contamination might include unusual odors, color changes, or sediment.

### What to Do:

- **Check for Clarity**: If the serum was clear before freezing and is now turbid, it’s worth investigating further.

- **Smell Test**: A foul odor can be an indication of bacterial contamination.

- **Microbial Testing**: If you're concerned about contamination, consider performing a microbial test or using a sterility test to check for the presence of bacteria or fungi.

- **Usage**: If you determine that the turbidity is due to harmless causes like protein precipitation or lipid separation, you may still be able to use the serum after gentle mixing. However, if there's any doubt regarding contamination, it's safer to discard the serum.

### Conclusion

If the turbidity persists or you have concerns about the quality of the serum, it may be best to consult your supplier or discard it and obtain a fresh batch to ensure the integrity of your experiments.

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