ABSTRACT Human diploid fibroblast-like cells were maintained in an arrested, essentially non-mitotic state for extended periods of time in culture by lowering the serum concentration in the medium from 10 to 0·5 %. These arrested cells re-entered the proliferative state when subcultivated in medium containing 10 % serum. The morphological distribution and enzymic activities associated with the Golgi complex were examined during growth, arrest, and recovery. Cells grown in medium containing 10 % serum possessed a well-developed Golgi complex consisting of parallel arrays of membranes and associated vesicles. Galactosyl transferase activity was highest after 3 days growth (17·5 ±5·0 nmol galactose transferred/45 min/mg protein) and decfined to 9·8 ± 30 at day 7. When the serum concentration was reduced to 0·5 %, Golgi complexes were rarely observed by electron microscopy and galactosyl transferase activity was further reduced to 4·6 ± 1·2 and 3-2 ± 1·4 after 3- and 7-day arrests, respectively. Arrested cells subcultured into medium containing 10 % serum recovered from the arrested state and ultrastructurally resembled cells continuously cultured at the higher serum level. Numerous Golgi complexes reappeared and galactosyl transferase activity increased to 13·0 ± 3·34 days after subcultivation. These results indicate that the Golgi complex can be experimentally manipulated in human diploid fibroblast-like cells in a manner which may be useful for the study of the biogenesis of this organelle.