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Thirteen terrestrial psychrotrophic bacteria from Antarctica were screened for the presence of a thermolabile ribonuclease (RNAase-HL). The enzyme was detected in three isolates of Pseudomonas fluorescens and one isolate of Pseudomonas syringae. It was purified from one P. Fluorescens isolate and the molecular mass of the enzyme as determined by SDS-PAGE was 16 kDa. RNAase-HL exhibited optimum activity around 40 degrees C at pH 7.4. It could hydrolyse Escherichia coli RNA and the synthetic substrates poly(A), poly(C), poly(U) and poly(A-U). Unlike the crude RNAase from mesophilic P. Fluorescens and pure bovine pancreatic RNAase A which were active even at 65 degrees C, RNAase-HL was totally and irreversibly inactivated at 65 degrees C.