The phenotype and functions of T lymphocytes and of natural killer (NK) cells have been investigated in four children and five adults from three Italian families infected with HIV (LAV/HTLV III). The results show a heterogeneous pattern of immunological derangements involving distribution of T and natural killer subsets, proliferation in response to T cell mitogens and natural killer activity. However, all infected patients tested showed a very low or absent phytohaemagglutinin induced interleukin-2 production regardless of age and clinical conditions, while concanavalin A-induced interleukin-2 production was within the normal range. The impaired interleukin-2 production in response to phytohaemagglutinin in some patients is not related to phytohaemagglutinin-induced proliferation, to clinical conditions or to a defective distribution of T cell subsets. These results suggest that, in our patients, both adults and children, HIV (LAV/HTLV III) has an "early" tropism for a subset of T cells involved in interleukin-2 production.