The immunological cross-reactivity of heterogeneous acid phosphatase isozymes from different human tissues has been studied using monospecific antisera prepared against four homogeneous acid phosphatases. The enzyme characterized as tartrate-inhibitable, prostatic acid phosphatase is also found to be present in leukocytes, kidney, spleen, and placenta. The tartrate-inhibitable (liver) lysosomal enzyme is also found in kidney, fibroblasts, brain, placenta, and spleen, but it is not detectable in erythrocytes and prostate. In several tissues, 10-20% of the tartrate-inhibitable enzyme is not precipitated by any of the antisera used; an exceptionally high amount (54%) of such an enzyme is present in human brain. Antiserum against a low molecular weight tartrate-resistant liver enzyme (14 kDa) does not crossreact with the erythrocyte enzyme. (10-20 kDa). All other tissues except placenta, prostate, and fibroblast cells show a cross-reactivity with the 14-kDa acid phosphatase antiserum. Thus, the low molecular weight human liver acid phosphatase is distinct from the erythrocyte enzyme, and there are also at least three different tartrate-inhibitable acid phosphatases in human tissues. Chromosomal assignments have been made for only two of the (at least) five acid phosphatases that are present in adult human tissues.