Human alpha- and gamma-interferons (IFNs) have the capability of enhancing the expression of major histocompatibility complex-class I and class II (MHC-I and MHC-II) as well as other surface tumor associated antigens (TAA) on cancer cell lines. This capability, associated to the anti-proliferative effect observed both in vitro and in vivo, was the basis of the extended use of IFN in cancer patients. Despite the great expectations, several solid neoplasms resulted resistant to treatment with IFNs. In order to analyze the actual effects of IFNs on cancer, we treated primary cultures of different histotypes of cancer cell, which are well known to differ significantly from established cell lines, with escalating doses of alpha-IFN. In this paper, we demonstrate that a variable proportion of culture was able to potentiate the expression of MCH-I and MCH-II molecules on the surface. This finding suggests that a clear functional heterogeneity was present in primary cultures of cancer cells, that strictly reflected the in vivo situation. In addition, it is of note that in some histotypes, "natural" lymphoblastoid alpha-IFN was more effective than recombinant human alpha-IFN in the induction of this capability.