The work presented proposes origins of the extensive range of observations concerning changes in bone joints associated with rheumatoid arthritis, osteoarthritis and arthritis urica. These changes are shown to originate with alterations of cell reactions involving four basic cell biochemicals. The proposals allow explanations of, for example, the link between rheumatoid arthritis and anaemia. It is also shown that performic acid is a presently unrecognised cause of degradation of tissue in bone joints. Suggestions are made as to means of control of these changes.