Human B-lymphoblast cell lines established from peripheral blood after infection with retroviruses of the simian sarcoma/simian sarcoma associated virus (SSV/SSAV)-gibbon ape leukemia virus (GALV) group generally do not produce detectable virus, even though viral RNA and protein are expressed. We show here by Southern blot and liquid molecular hybridization experiments that most of these nonproducer cultures not producing virus contain apparently complete proviral DNA sequences which lack detectable rearrangements, indicating that restriction of virus replication is not due to obvious proviral lesions. The provirus is often present in only a fraction of the cell population, although the proviral copy number is variable and increases during extensive tissue culture or passage through nude mice. The proviral DNA is integrated into multiple heterogeneous sites in these cells, indicating a polyclonal population. A few of these cell lines have been cloned. Clonal lines have so far originated only from cells which contain proviral DNA and express viral RNA, although they still do not produce complete or infectious virus. Southern blots show that these cells are clonal in origin and contain a complete provirus with two LTRs arranged in the normal orientation. Two viral mRNA species are expressed which are identical in size to those from virus-producing cell lines. The smaller (presumably env) mRNA, like that from virus-producer lines, is a spliced transcript which contains both 5′- and 3′-derived sequences. These data strongly suggest that these cells restrict viral replication at some stage later than mRNA synthesis, possibly at the level of protein synthesis or processing.