The quark parton model is applied to the deep-inelastic region of single-particle inclusive lepton-hadron processes. The current-fragmentation region is mainly discussed. Various conclusions concerning experimentally observable quantities such as single-particle distribution functions and average multiplicities are reached. Sum rules such as the Bjorken and Adler sum rules are generalized to this case. The average multiplicities of current-fragment pions in neutrino reactions are found to be independent of the target and of x. They are related to electroproduction multiplicities. A relationship is established between the x → 0 limit of the latter, and pion multiplicities in e+e- annihilation. Assuming SU(3) invariance, relations among π and K multiplicities are derived.