Series I was released in 1991 by Toy Biz. It was the largest assortment of figures to date for this line (9 figures). However, a sister line, X-Force, was quickly developed on the heels of the success of the original line. In addition, this original assortment included vehicles (for Magneto and Wolverine) and playsets (Wolverine Combat cave and Cyclops Lightforce Arena). The original 1991 playsets were re-released in 1994.