Over the past two years, antisense RNA technology has been developed in plants in several experimental systems. Progress reported in the literature will be reviewed. Special emphasis will be placed on experiments in which antisense RNA has been used to generate novel tomato mutants through the downregulation of polygalacturonase (PG), a major cell wall hydrolase. This work has shown that antisense RNA inhibits PG expression specifically without affecting the expression of other genes. The antisense gene and its phenotypic expression are stably inherited. The inhibitory effect of an antisense gene can be enhanced by increasing the copy number of the gene. The interaction of the antisense gene with its target is only seen when the target gene is expressed. Antisense RNA technology has been used to clarify the biochemical function of polygalacturonase and its role in tomato fruit softening. Recently, this approach has also been applied to elucidate the biochemical role of another gene from tomato, pTOM13, whose function was previously unknown. Thus, antisense RNA technology can now be used to generate novel plant mutants that will make possible the definition of the biochemical and biological role of genes whose functions are otherwise unknown.