This question often arises when people read Leviticus 19:19 and Deuteronomy 22:11. Today we would call this a "consumer protection law." Notice that these verses contain the basic principle that materials of widely differing character and texture are not to be combined. On the other hand, these verses allow a number of combinations that are within God's laws.
Today's garments are made of two basic kinds of natural fibers. The first is plant cellulose fiber, from which fabrics such as linen and cotton are produced. The second is animal protein fiber such as wool and silk. Because these kinds of fibers differ markedly in strength, washability, absorption, and so forth, they should not be mixed.
However, a garment made of a combination of cellulose materials—a mixture of cotton and linen, for example—is acceptable because the fibers are basically similar. For the same reason, mixtures of protein fibers (wool, mohair, silk, and so on) are acceptable.
What about the mixture of synthetic, man-made fabrics, such as Dacron, nylon, polyester, and rayon, with either cellulose or protein fibers? Many have not realized that a combination of synthetic and either plant or animal material does not necessarily break the biblical principle. Synthetic materials are usually made to have essentially the same characteristics as the natural fibers. Otherwise, they would not mix well. The stronger fibers would cut and tear away from the weaker ones or would not combine well in other ways. In other words, it is perfectly acceptable to manufacture fabrics from a combination of fibers which are naturally or artificially compatible with one another. It is the mixture of fibers with markedly differing qualities which this biblical principle concerns.
It should be noted that such combinations produce a cheaper garment, with respect to quality, than one made with the best grades of pure fibers. On the other hand, a fabric made from low-grade, natural fibers is usually improved by the addition of compatible man-made fibers. Any good tailor or seamstress knows that the best quality clothing is made from 100 percent wool, cotton, and so forth. Nevertheless, one need not throw away or destroy clothing which may be of lower quality or a wrong mixture. Wearing such materials is not a sin in itself. Rather, God does not want manufacturers producing shoddy materials in order to take advantage of their customers.
A wise principle to follow in selecting either a pure or mixed garment is to purchase the best quality one can afford—it will last longer and fit better than inferior, less expensive clothes. The primary reason to do this is to honor and glorify God in what we wear, especially if the clothing is to be worn primarily for church services. However, it is not wise to go into debt buying better quality than one can afford.