An interspecific hybrid between Lycopersicon esculentum (♀) and L. peruvianum has been raised by embryo rescue in vitro and used to confirm the presence of a new S-allelic specificity in its inbred L. peruvianum parent, a plant derived by enforced bud self-pollination of a self-incompatible clone with the genotype S 1 S 2. The inbred plant showed breeding behavior characteristic of both S 2 and a second specificity which was not S 1, S 2, S 3 or S f. Two-dimensional gel electrophoresis of stylar proteins, however, showed only a single typical S-associated component with the Mr and pI characteristic of S2. The alteration in specificity, therefore, was not associated with a detectable change in an S-associated protein. The F1 interspecific hybrid showed intermediacy of vegetative and reproductive characters, relatively high fertility and full self-incompatibility. Backcrossing to L. esculentum produced only abortive seeds requiring embryo culture. Backcrosses to L. peruvianum produced a very low proportion of filled germinable seeds. Pollen of the hybrid showed superior viability and tube growth rate compared with pollen of the two parent plants.