Alpha- and beta-momorcharins are ribosome-inactivating proteins present in the seeds of the bitter gourd (Momordica charantia). Both of them possess ribonuclease activity which may account for some of their biological properties. However, the activity is weak and hence it is important to confirm that the ribonuclease activity observed is not due to any contamination. To this end, the ribonuclease from the seeds of M. charantia (RNase-MC) was purified and compared with the ribonuclease activity of the momorcharins. Purification was achieved by ion-exchange chromatographies on DEAE-cellulose, SP-Sepharose and Mono-S. RNase-MC had a molecular mass of 22 kDa. It acted on tRNA to release acid-soluble UV-absorbing species with a pH optimum around 6.0-6.5. When polyhomoribonucleotides were used as substrates, it was found that RNase-MC acted preferentially on polyU but exerted much weaker activity on polyC, polyG and polyA. Chromatographic analysis of the reaction product indicated that mono- and oligo-ribonucleotides, but not free base, were generated from polyU, suggesting that the enzymatic action involved ribonucleolytic cleavage. RNase-MC exhibited a much more potent (at least 1000-fold higher) ribonuclease activity than alpha- and beta-momorcharins. RNase-MC, alpha-momorcharin and beta-momorcharin were separable on Mono-S, indicating that the ribonuclease activities present in the three proteins were distinct entities.