Thymus-dependent (T) lymphocytes from (2 x 13)F1 hybrid guinea pigs immunized to ovalbumin (OVA) in complete Freund's adjuvant can be stimulated to proliferate in vitro by antigen-pulsed peritoneal exudate cells (PECs) derived from either strain 2 or strain 13 donors. In this communication, we show that the population of primed F1 T lymphocytes which can be activated by antigen-pulsed strain 2 PECs is largely independent of the population of cells that can be activated by antigen-pulsed strain 13 PECs. This was demonstrated by both positive and negative selection procedures. In the former, T lymphocytes from OVA-primed (2 x 13)F1 donors were enriched by initial culture with OVA-pulsed strain 2 or strain 13 PECs for 1 wk. Cells selected by culture with OVA-pulsed strain 2 PECs responded well to OVA-pulsed strain 2 PECs and poorly to OVA-pulsed strain 13 PECs. If positive selection had been carried out with OVA-pulsed strain 13 PECs, the selected F1 T cells responded well to OVA-pulsed 13 PECs and poorly to OVA-pulsed 2 PECs. Negative selection was achieved by short term culture with antigen-pulsed PECs and by eliminating proliferating cells by treatment with bromodeoxyuridine and light. This procedure demonstrated that the population of primed F1 T lymphocytes which are responsive to OVA or to purified protein derivative of tuberculin can be divided into subpopulations uniquely responsive to antigen on either strain 2 or strain 13 PECs. Evidence was presented to indicate that this selective responsiveness was not the result of the action of alloantigen-specific suppressor cells. The results are considered in terms of current concepts of the genetic and molecular regulation of the interaction of PECs and T lymphocytes.