Normal peripheral human white blood cells (WBC) plated in agar culture generate colonies of blastoid cells when stimulated in vitro by phytohemagglutinin (PHA). Aggregates of PHA transformed cells are first visible by day 3-4. By day 7, the morphology of colonies becomes typical as clear discoid raspberry-like aggregates of 50-200 cells or more growing at or near the surface of the agar plate. Their dense arrangement and the coexistence within individual colonies of large and smaller cells are 2 main characteristics of these colonies. Wright-Giemsa stained preparations from centrifuged colonies show cells at different stages of transformation. These cells are PAS [periodic acid Schiff] and peroxidase negative. They do not bear surface immunoglobulins, but most of them form rosettes with sheep red blood cells. These colonies are probably made of T[thymus-derived]-lymphocytes. In view of the number of isolated transformed cells which do not form colonies, it is possible that the cells which generate these colonies may represent a subset of PHA sensitive cells.