We prove that if A is a prime non-commutative JB*-algebra which is neither
quadratic nor commutative, then there exist a prime C*-algebra B and a real
number lambda with 1/2 < <lambda> less than or equal to 1 such that A = B
as involutive Banach spaces, and the product of A is related to that of B (
denoted by o, say) by means of the equality xy = lambdax o y + (1 - lambda
)y o x.