The purpose of this study was to investigate the relationship of a set of factor-analytically derived ability measures to performance at different stages of practice in a learning set task employing concept problems, and to consider possible constructs to explain these relationships. Twenty-six problems, each of which required the identification of a four-dimensional conjunctive concept, were administered to a sample of 145 Princeton University undergraduates. To account for the complex learning involved in this task, a conceptual model involving information-processing constructs was developed. Three higher-order processes were postulated: a problem analysis process; a search process; and an organization process. A battery of 30 mental tests was administered to the same sample of students. The tests were selected as measures of factors chosen for their relevance to the model. Ten mental ability factors were thus defined: three reasoning factors, two flexibility factors, three memory factors, and two visual speed factors. Verbal reasoning was the most highly related of all mental abilities to early success in obtaining the correct structuring rules for both positive and negative concept examples. A new "chunking memory" factor was strongly related to learning performance, especially during early stages of practice. Memory Span interacted with Induction and Symbolic Flexibility on number of problems solved, while Spatial Scanning interacted with Memory Span. This study was seen to have implications to the problem of providing an empirical base for the construct validation of both mental tests and theories of complex human learning.