To test the influence in primary MLC of the newly PLT defined determinant PL3 A (Termijtelen et al. 1980), 11 Dw3/DR3 homozygous individuals were tested in an MLC matrix. Two groups of mutually negative cells were defined. The results appeared to correlate with the presence of PL3A. Cells from group 2 were stimulated by cells from group 1, but the reverse reactions were negative. A genetic model for this “one way” stimulation, observed in MLC as well as PLT, is discussed.We suggest that PL3A, which causes strong stimulation in PLT, also plays a role in primary