This study examined how professional learning communities develop, using cross-subject teams within the context of HUMANITAS, a program designed to promote teachers' professional growth and effectiveness by creating a voluntary "community of learners" for teachers and students. Data were gathered from teacher interviews, team observations, teacher questionnaires and site records from thirty-five interdisciplinary teams (68 teachers in 8 secondary schools) in the diverse and urban Los Angeles Unified School District. Findings indicated that most teachers found participation in HUMANITAS to be a positive force facilitating collaboration and interdisciplinary teamwork, although the collegial, collaborative spirit that developed among teachers in the HUMANITAS teams did not extend to other teachers or departments in the same school. Since only those teams which successfully implemented HUMANITAS are discussed in this study, a call is made for further research on factors impeding implementation of such programs, as well as reasons why many schools never initiate participation in such programs at any level. (Contains 35 references and 8 tables.) (PB)