PHA-induced T lymphocyte colony formation in semi-solid agar culture was studied in mononuclear cells (MC) and non-adherent cells (NAC) from the blood of patients with infectious mononucleosis (IM). Colony formation expressed as number of colonies per 10(6) E-RFC or as number of colonies per ml of blood was depressed by about 90% during the first weeks of disease, but returned to normal levels during the convalescence period. Addition to the agar culture of conditioned medium prepared from adherent blood MC or normal donors partly restored colony formation by both MC andNAC from patients with IM in the acute stages, suggesting a subnormal production of conditioning factors by cocultured adherent cells. In line with this finding adherent cells from patients with acute disease failed to produce a conditioned medium which optimally supported the growth of T lymphocyte colonies from NAC of normal donors. When mononuclear cells from patients with IM were mixed with normal donor lymphocytes prior to agar seeding, colony formation by the normal cells was reduced by 10--65%. It is concluded that mononuclear cells from patients with IM have a reduced capacity to form T lymphocyte colonies in agar medium. This reduction possibly reflects a lack of production of colony stimulating factors from monocytes, but also increased activity of T lymphocyte colony suppressor cells.