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This study was performed to evaluate a washed-cell, thick-smear antigen for use in the indirect fluorescent-antibody test for malaria. Thick-smear antigens of Plasmodium vivax, P. falciparum, and P. brasilianum were prepared from parasitized erythrocytes washed free of soluble serum components by several changes of phosphate-buffered saline solution, pH 7.2. Serum samples both positive and negative for malaria antibody were tested with blind and double-blind techniques. The lowest significant diagnostic titer was found to be 1:16. At this dilution, false positive rates were less than 0.5%, false negative rates were below 5%, and the range of titer reproducibility was plus or minus one fourfold dilution. Use of the modified antigen had certain advantages over the conventional whole-blood, thin-smear antigen. Binding of the antigen donor's antibody to his plasmodia is avoided. This effect, previously attributed to an in vivo reaction, seems to occur in vitro. Reading is facilitated by the large number of plasmodia per field.