The effects of in vivo treatment with recombinant human IFN-alpha 2a (rIFN-alpha 2a) on the distribution of T cell subpopulations were examined in 21 patients with renal cell adenocarcinoma, using two-color flow cytometry with anti-Leu8 in combination with anti-Leu2 and anti-Leu3. Other parameters indicative of immune status, such as the number and percentage of total (CD3) T cells, in vitro proliferation to mitogen, and spontaneous immunoglobulin secretion, were also measured, prior to the initiation of treatment with rIFN-alpha 2a, and during treatment. Total T cell number decreased after treatment with rIFN-alpha 2a, to a low of 54% of mean pretreatment values after 4 weeks. The CD4/CD8 ratio did not change appreciably following treatment with rIFN-alpha 2a. However, the number of Leu8-negative T cells, within both the CD4 and the CD8 T cell populations, decreased more than the number of Leu8-positive T cells. An increase in spontaneous immunoglobulin-secreting cells followed treatment with rIFN-alpha 2a.