Some great advice here. I echo much of what Sean said above, if you are looking to potentially retain them as a client, then lawyers really are a last resort. I would get them on the phone, and learn why they are not paying. These can be touchy situations, so adding a human touch can be helpful. They may be tight on cashflow and you can offer a payment plan, or a 3-6 month deferral. If they are going bankrupt, save the legal fees and get on the creditor list. Do these clients have a history of paying on time? Understanding their payment history may guide your approach. If this doesn't work, they maybe go the demand letter route with a lawyer. I would think twice about going any further with a lawyer, since fees on 2 seperate suits may quickly exceed the $100K owed.

@phil - two key pieces of information you didn't include... 1. Are they a large company, PE backed, or small privately owned business. You can't treat all customers equally - certain approaches work better depending on the type of customer. 2. Do they not want to pay their bill because they are having cash flow issues, or do they have a complaint and are they claiming that the product or service did not meet specs, requirements?