We examined the possibility that one mechanism for controlling HLA-DR alpha gene expression involves the alteration of chromatin structure. Chromatin structure was analyzed by measuring the susceptibility of DR alpha genes in intact nuclei to nuclease treatment. We first examined a somatic cell hybrid of a T-lymphoblastoid cell line (LCL) and a B-LCL, since the DR alpha gene, which is inactive in the T-LCL parent, is expressed in the hybrid, thus providing a system to study DR alpha gene induction. The hybrid line 174 X CEM.T1 contains and expresses solely the DR alpha gene from the T-LCL parent, since the DR alpha gene from the B-LCL parent, 174, is deleted. Using cytoplasmic dot blot analysis and RNA-DNA Northern hybridization, we detected DR alpha-specific transcripts in the hybrid, but not in the parental lines, indicating activation of the DR alpha gene in the hybrid. The transcribed DR alpha gene from the hybrid was compared with the untranscribed gene from the T-LCL parental line, and an association between DR alpha gene expression and increased sensitivity to DNase I was observed. A switch in the chromatin structure of the DR alpha gene from a closed to an open configuration apparently occurred in this hybrid. Such a change is associated with DR alpha gene expression. Comparison of a DR-positive B-LCL and an isogenic DR-negative T-LCL also showed that the chromatin of the former is more sensitive to DNase I digestion. There were no restriction enzyme fragment length differences between the DR alpha genes from 174 X CEM.T1 and CEMR, indicating that the process of somatic cell hybridization did not result in DNA rearrangement or translocation.