. "The ultrasonic contrast agent of claim 4, wherein the polyglutamic and aspartic acid esters or amides have side functions having formulae (CH2)n COO--CHR1 COOR (I), or (CH2)n COOCR1 R2 --O--COR (II) or (CH2)n CO(NH--CHX--CO)m NHCH(COOH)--(CH2)p COOH(III), wherein R is an alkyl or aryl substituent; R1 and R2 are H or lower alkyls, or R and R1 are connected togethe" . . .