A protocol for analyzing the activity structure of a classroom lesson was developed and field tested. The protocol can be completed on site by a person serving as both observer and analyst. Validity of the protocol was established by reference to the large body of qualitative research on activity structure and the analysis of teaching. The reliability of the protocol was assessed by calculating the percentage of inter-observer/analyst agreement for 47 lessons observed in 24 classes by pairs of observers/analysts. Observers were nine graduate students in the School of Education of a large research university who had received instruction in the activity structure approach. Initial inter-observer agreement ranged from 57% to 96.5%. When added to the percent of agreement that resulted from consensus seeking, overall agreement reached 97.6%. Results of an exploratory analysis of the content level, grade level, and subject area, and the inter-observer agreement indicate the utility of the activity structure analysis for data collection. Two tables and one figure contain information from the study. Appendixes contain: (1) directions for segmenting the lesson and mapping activity structure; (2) the activity structure analysis codebook; and (3) the analysis form. (SLD)