NEW MATERIAL: A compound of formula I [X is H or NHR1 (R1 is H or an amino-protecting group); R is (substd.) spiro-fused or bridged bicyclic group optionally having O, N or S].

EXAMPLE: 6β[Z-[2-(2-Aminothiazol4-yl)-3(bicyclo[2,2,1]hept2-yl)]propeneamido]- penicillanic acid.

USE: Antibacterial agents which are highly stable to β-lactamase.

PROCESS: A compound of formula II (the amino group may be substituted by an acyl-forming group; R3 is H or a carboxyl-blocking group) is treated with an acylating agent derived from an acid of formula III (Y is formula IV).