A sensitive, specific, and reproducible ELISA which incorporated an oocyst soluble antigen was used in order to detect specific Cryptosporidium immunoglobulins G and M in Italian HIV-positive patients. The soluble antigen was prepared from purified oocysts collected from experimentally infected calves. The serum working dilution was established at 1 in 50. Of parasitologically positive and HIV-positive patients, 95% showed specific IgG or IgM or both in their serum. Of HIV-positive patients, some of whom had diarrhoea of uncertain aetiology, 15.8% were found to have specific IgG. Both specific IgG and IgM were detected in the serum of an HIV-positive patient 1 year before the shedding of oocysts in the faeces. Sixteen (5.3%) of 300 presumed healthy people, positive for specific IgG, were all IgM-negative. Any significant cross-reactions with other parasitic infections were not observed. With a serum dilution greater than 1 in 25, only a low degree of positivity was observed with samples highly IgG-positive for Toxoplasma species. The ELISA showed a higher than expected prevalence of Cryptosporidium infection in Italian HIV-positive patients.