After stimulation of guinea pigs with estradiol to increase their Kurloff cell number, spleen imprints were prepared in order to detect non‐specific acid phosphatase (AcPase) activity by light microscopic cytochemistry using naphthol AS‐BI phosphate as substrate and pararosanilin or fast garnet GBC as coupler. For ultracytochemistry, Kurloff cells were prepared from spleens by filtration through a homogeneizer screen followed by repeated centrifugation. AcPase and trimetaphosphatase activities were tested using beta‐glycerophosphate, cytidine‐5′‐monophosphate and inorganic trimetaphosphate as substrates. Significant enzymatic activities were demonstrated with all the substrates used in the cytoplasmic inclusion body of the Ku