PHA-induced colonies were obtained from peripheral blood mononuclear cells (MC) grown in agar-medium. When the colonies were harvested from mass cultures, pooled as single cell suspensions and plated again in presence of PHA, they failed to generate new colonies unless they were seeded on an underlayer containing uncultured blood MC. Cytogenetic studies indicate that most secondary colonies were derived from primary colonies. Autologous as well as heterologous feeder cells were able to promote the growth of secondary colonies. No granulocyte (G) or macrophage (M) colony formation was observed in secondary cultures. These experiments show that the progenitors of PHA-induced colonies differ from G or M CFCs and that they are still detected in these colonies which contain 82 +/- 12% T-cells. In contrast, colony formation requires the presence of factor(s) provided by cooperating cells (CC) which are no longer detected in primary colonies and this is associated with a depletion in non-T elements from the initial MC population.