The previously reported three‐parameter stress function has been applied to natural rubbers similar to those used for engineering purposes. These rubbers were selected to cover a harness range from 30 to 80 British Standard Degrees. A suitable choice of values for the parametersA, B, and &bgr; was found to give a good description of the stress‐strain characteristics in pure shear, and positive and negative stretch squeeze. This agreement applied up to the maximum possible strains. Unique functional relationships were found to exist between the three parameters and rubber hardness. Theoretically, it would be possible to reduce the three‐parameter stress function to a single‐parameter stress function. As previously reported, this confirms and hence is indirectly confirmed by Blackwell's results [Trans. I. R. I.28, 75 (1952)].