Acidic chitinases (EC 3.2.1.14) were isolated and characterized from 4-week-old nonembryogenic Citrus sinensis L. Osbeck cv 'Valencia' callus tissue. The enzymes were purified using size exclusion, anion exchange, and chromatofocusing HPLC techniques. Eleven isoforms were isolated with M(r)s between 26,000 and 37,400. Eight of the isoforms were purified to homogeneity, and all but one cross-reacted with a polyclonal antibody raised against a basic class I potato leaf chitinase. The isoelectric points (determined by chromatofocusing) were from pH 4.5 to 5.4. All hydrolases degraded chitin and four were capable of hydrolyzing solubilized shrimp shell chitosan suggesting they may be chitosanases (EC 3.2.1.99). Apparent chitosanase activity generally decreased with decreasing acetylation of the chitosan (i.e. from 20% to 0% acetylation). The chitinases and chitinases/chitosanases are predominantly endochitinases. Chitosanase activity was optimal at pH 5 while the pH optimum for chitinase activity ranged between pH 3.5 and 5.5. The chitinases and chitinases/chitosanases were stable up to 60 degrees C and showed their highest enzyme activity at that temperature. N-terminal sequences were obtained on three of the isoforms. One of the isoforms was identified as a class II chitinase and the other two as class III chitinases.