Additional parameters for the chloramphenicol acetyltransferase (CAT) activity in spores of S. griseus are substantiated. A linear increase in activity was observed with increasing spore number up to a concentration of 5 x 10(10) spores/ml. Similarly an increase of the chloramphenicol concentration up to 500 mug/ml increased the activity. However, a drastic decrease in activity was noted above this level suggesting inhibition of the enzyme by the substrate. The CAT activity in the spores was highly influenced by the pH of the medium reaching a maximum at pH 6.5. This may suggest that CAT is apparently located to the outer surface of the spores and therefore very sensitive to variations in pH of the medium. The CAT showed a marked specificity for D-threo and D-erythrochloramphenicol, while no activity was observed with L-isomers. The enzyme acetylates D,L-erythrodechlor-chloramphenicol with a yield of 45% as compared to the D-threo parent antibiotic. While the tyrosinase characteristics (melanin formation) of S. griseus was eliminated by acriflavine or ethidium bromide treatment the CAT characteristic was persistent. The melanin negative variants retained all otherproperties of the parent strain including the production of antimicrobial agents; and revertants were not detected. The results suggest that the tyrosinase determinant gene is apparently located on an extrachromosomal element (plasmid). On the other hand, the location of the gene for CAT is not assigned yet. The nature of CAT in growing cells and the spores of S. griseus was investigated. The results show that CAT accumulated during the sporulation phase or the vegetative growth is inducible in nature; therefore the morphogenetic sequence in the strain bears no influence on CAT induction.