5(6)-methyl substituted fluorescein derivatives

Fluorescein conjugates of the following formula XIX or XX:

• ##STR47## wherein R is hydroxy or protected hydroxy and X is chosen from the group consisting of --N(H)n--, --O--, or --S--;
• wherein n=0 or 1, to satisfy the nitrogen valency;
• wherein A is a spacer group consisting of from 0 to 50 carbon atoms and heteroatoms, including not more than ten heteroatoms, arranged in a straight or branched chain, saturated or unsaturated, with the provisos that not more than two heteroatoms may be directly linked in the sequence --X--A--M--Q, the sequence --X--A--M--Q cannot contain --O--O--, that when X=--N(H).sub.n -- in the sequence --X--A--M--Q, when n=0 then N--A taken together may comprise a ring of not more that 6 atoms;
• and that branchings may occur only on carbon atoms;
• wherein M is a linking group selected from --C(.dbd.O), --NH--, --O--C(.dbd.O)--, --N(H)--C(.dbd.O)--, --N(H)--C(.dbd.S)--, --S--, --P(.dbd.O)(O--)-- and ##STR48## and wherein Q is a conjugation partner.