Aquila has developed a new strings they call Nylgut, which has all the good qualities of old fashioned gut strings. I put them on a Uke and they were great. (See Uke Talk 9/29 Pauline Leland post "Nylgut Strings for a Uke") Tone and resonance was outstanding. So, I took the plunge and put a set on the Martin.
If you have an acoustic guitar I highly recommed you give them a try. The difference is remarkable.
They are expensive at $22.00 a set. If you want the USA distributor name send an e-mail
I've ordered some for guitar, along with some for uke, and the order is held up until the uke strings arrive from Italy. Aquila is selling uke strings like hotcakes ever since some clever fellow in Japan discovered how good they are. Dealers can't keep them in stock. At least mine can't.