We consider families (L t , t # T) of positive linear operators such that each L t is representable in terms of a stochastic process starting at the origin and having nondecreasing paths and integrable stationary increments. For these families, we give probabilistic characterizations of the best possible constants both in preservation inequalities concerning the first modulus and in preservation of Lipschitz classes of first order. As an application, we compute such constants for the Bernstein, Sza sz, Gamma, Baskakov, and Beta operators.1998 Academic Press