Describes a method of item factor analysis based on Thurstone's multiple-factor model and implemented by marginal maximum likelihood estimation and the em algorithm. Statistical significance of successive factors added to the model were tested by the likelihood ratio criterion. Provisions for effects of guessing on multiple-choice items, and for omitted and not-reached items, are included. Bayes constraints on the factor loadings were found to be necessary to suppress Heywood cases. Applications to simulated and real data are presented to substantiate the accuracy and practical utility of the method. (PsycINFO Database Record (c) 2000 APA, all rights reserved)(unassigned)