As most people know, a garment made of wool and linen, known as shaatnez, is forbidden. And if you were to tie wool tzitzit onto a linen garment, or linen tzitzit onto a wool garment, it would also be considered shaatnez.
However, there is one special exception: the Torah permits us to tie wool tzitzit with techelet onto a linen garment. However, if you have any slight doubt that your techelet dye may not be the type of dye delineated in the Torah, you won't want to take chances creating shaatnez. Therefore it came as a bit of a surprise for me to see the following inquiry in my inbox, Sunday morning, bright and early.
Can you do linen zizit with the wool techelet string for me? Thanks -JMA
Quite a brave soul!
I explained to Mr. JMA that we don't sell any linen tallit katan garments. Almost nobody makes them because the minhag, brought down in the Shulchan Aruch, is not to use a linen garment nowadays. According to my understanding, even if more people are now using techelet, and even if you hold that Murex trunculus is without a doubt the proper dye, the minhag would not change, because it would be a stumbling block for those who wear wool.
Imagine if you went to your local Judaica shop and saw all sorts of tallit katan garments -- wool, cotton, linen and cotton/synthetic blends. Then on another shelf you saw tzitzis strings -- wool in machine-spun, hand-spun and niputz lishmah, both all-white and techelet, as well as linen hand-spun all-white tzitzit. Someone could easily wind up wearing 100% shaatnez unknowingly if they are not aware of the issue and on the alert.
So I suppose the logic is that until techelet becomes very widespread or even prevalent, we look at the big picture and take extra measures to keep people out of trouble.
If I'm not mistaken, rabbanim might be amenable to a linen garment under certain circumstances, such as someone who is allergic to cotton and wool, but not to linen. We wrote a bit more on the topic here.