Human extracellular ribonucleases (RNase), together with other members of the mammalian RNase superfamily, can be classified into four different enzyme types on the basis of their structural, catalytic and/or biological properties. Their occurrence and main distinctive features have been described, and catalytic differences (action on single- and double-stranded RNAs, dependence of enzyme activity on pH, ionic strength and cations, and hydrolysis of cyclic nucleotides) have been comparatively analyzed and discussed. In addition, some data considered here concerning human nonpancreatic-type RNases may support the suggestion [Chuchillo et al. (1993) FEBS Lett. 333, 207-210] that the enzyme 'ribonuclease', presently classified as 'hydrolase', should be reclassified as 'transferase'.