This article reports the results of a study to determine the effects of austenite stability, with respect to the strain-induced transformation to martensite, on the formability of 300 series stainless steels. The effects were evaluated as a function of alloy content, deformation temperature, and deformation rate. Three stainless-steel alloys with different nickel contents were evaluated as commercially cold-rolled and annealed sheet products. Tensile tests were performed at temperatures between −60°C and +125°C and at strain rates from 0.00167 s−1 to 0.167 s−1. The combined effects of strain, strain state, deformation-induced temperature changes, and strain rate are considered to explain the interrelationships between martensite formation and limit strains as observed in forming-limit diagrams.