If L and M are partially ordered vector spaces, then one can consider regular linear maps from L to M, i.e. linear maps which can be written as the difference of two positive linear maps. If the space L is directed, then the space L r (L , M) of all regular linear operators becomes a partially ordered vector space itself. We will mainly concern ourselves with the questions when the space L r (L , M) is itself a Riesz space and how, even if it is not a Riesz space, its lattice operations work. The so-called Riesz-Kantorovich theorem gives sufficient conditions for which L r (L , M) is a Riesz space and it also specifies the lattice operations by means of the Riesz-Kantorovich formula: if S, T ∈ L r (L , M) and x ∈ L with x ≥ 0 then the supremum S ∨ T in the point x is given byIt is still an open problem if whenever in a more general setting the supremum of two regular operators exists in L r (L , M), it automatically is given by the RieszKantorovich formula. Our main result concerns the special case where L is a partially ordered vector space with a strong order unit and M is a (possibly infinite) product of copies of the real line, equipped with the lexicographic ordering. It will turn out that under some mild continuity and regularity conditions the lattice operations on L r (L , M) are indeed given by the Riesz-Kantorovich formula, even though the space L r (L , M) is not necessarily a Riesz space.B W. M. Schouten