Samples of subretinal fluid (SRF) from patients with primary rhegmatogeneous retinal detachment of various duration were studied using standard enzymologic techniques. The protein content in SRF increased with the duration of detachment. The activities of lysosomal enzymes in SRF, which were acid phosphatase, beta-glucuronidase, and cathepsin D, also increased with the duration of detachment. Especially cathepsin D, which was not detected in serum, was present in SRF. The activity of a nonlysosomal enzyme, i. e., lactic dehydrogenase, which was used as a marker of cell disruption and of serum transudation was the same as that of serum. Because lysosomal enzymes are known to be able to degrade cells and tissues, the enzymologic analysis of SRF contributes to our understanding of the pathology of primary rhegmatogeneous retinal detachment.