Jung's typology implies that the attitudes and functions are (a) stable; (b) categorical or qualitatively dichotomous; (c) interacting; and (d) giving rise to different compounds of surface traits. In addition, it implies that type indeterminacy produces ineffective and maladjusted behavior. A program of studies using the Myers-Briggs Type Indicator found that (a) the type classifications had moderate temporal stability; (b) Indicator score distributions showed no marked evidence of bimodality; (c) with one exception, the regressions of other variables on Indicator scales did not change in either level or slope at the zero point of the Indicator scales; (d) the Indicator scales did not interact in relation to other variables; (e) the Indicator scales did not moderate the regression of other variables on one another; and (f) type indeterminacy was unrelated to measures of ineffective behavior and maladjustment. These findings offer little support for any of the structural properties attributed to the typology. Two possible interpretations of these negative findings were suggested: (a) Jung's typology is not consistent with the real world; or (b) the Indicator does not correspond to the theoretical formulation of the typology; that is, the Indicator does not operationally define the typology. Verification of these interpretations awaits further research.