Two siblings with different degrees of mental retardation, skeletal dysplasia, coarse facies, delayed speech, motor incoordination, recurrent respiratory infections, and immunological abnormalities, were found to have deficient alpha‐mannosidase activity. Cultured skin fibroblasts in one sib were markedly deficient in alpha‐mannosidase while all other lysosomal enzymes tested were within the normal range. The more severely affected sib came to autopsy and was found to have “washed‐out” appearing cortical neurons and marked histiocytosis effacing lymph node architecture and partially replacing the bone marrow. The post‐mortem brain and liver samples demonstrated a deficiency in alpha‐mannosidase relative to the elevations of other lysosomal enzymes. Although the patterns of abnormalities in the two cases closely match those of descriptions of “type II” and “type I” mannosidosis respectively, the variation should be due to genetic modifiers or environmental effects since the brothers must have shared similar alpha‐mannosidase mutations. Immunologic abnormalities present in the more severely affected sib suggest that the differential survival seen in mannosidosis types I and II may be due to differences in their immune systems.