RC4 is one of the most popular stream ciphers that generates pseudorandom words from pseudorandom permutations. In this paper we identify new bias for RC4 and its variants RC4A and VMPC, which are designed in a similar paradigm. Naturally, these biases provide new distinguishers for the pseudo-random keystream generated from these algorithms. In particular, our result provides the strongest distinguisher against VMPC. Although RC4A is of less practical interest, a lot of protocols use VMPC.