Technical Abstract: Castor oil is the only commercial source of ricinoleic acid and has numerous industrial applications. Among the factors limiting domestic production of castor oil is the presence of the toxin ricin and its less toxic homologue Ricinus communis agglutinin (RCA) in seeds. Although the sequences of ricin and RCA genes are known, their transcriptional expression patterns have not been distinguished due to their high degree of sequence similarity. As the information is critical for assessing success in developing a ricin-free castor crop using genetic silencing, we have designed a gene specific reverse transcription-polymerase chain reaction (RT-PCR) assay to examine the expression of the ricin and RCA genes in developing seeds. The results show that the ricin and RCA mRNA are highly abundant in seeds during the development of endosperm, and the expression pattern is similar to that observed in the Northern analysis. The RT-PCR results can be confirmed by a simple RT-PCR-based restriction fragment analysis.