Part I This is an investigation of differences in interpersonal behavior and learning in a sensitivity training laboratory between highly person-oriented and highly work-oriented participants (identified through the Person Description Instrument III). Second, it is a study of the effects of a training design that involves both heterogeneous and homogeneous training groups. It was expected, and confirmed at satisfactory levels of significance, that the person-oriented members would be seen as behaving more expressively and warmly and that they would be more comfortable and would feel stronger interpersonal ties within their homogeneous group than would the work-oriented members. It was expected, but with results approaching significance in the opposite direction, that person-oriented members would be seen as learning more than would the work-oriented members. It is hypothesized that the person-oriented group found the laboratory a kind of psychic home without much challenge, whereas the work-oriented members experienced "culture shock," and that this in fact pushed them toward change. Part II To test the learning model, data were examined from a laboratory in which each participant was assigned to a heterogeneous training group and to an experimental group composed in terms of preference for high, low, or moderate structure. The statistical and impressionistic data collected through member ratings of one another and through interviews with staff and participants strongly suggest that homogeneous groups do not provide the confrontation needed for optimum learning. The superiority of the mixed high-and low-structure (and more stressful) groups in terms of member learning suggests that feelings of completion, cohesion, and emotional satisfaction may not be the appropriate criteria for evaluating the impact of a training group experience.