EC50.calc<-function(coef,vcov,conf.level=.95) {

 # calculates confidence interval based upon Fieller's thm.
 # assumes link is linear in dose

        call <- match.call()

         b0<-coef[1]
         b1<-coef[2]
         var.b0<-vcov[1,1]
         var.b1<-vcov[2,2]
         cov.b0.b1<-vcov[1,2]
         alpha<-1-conf.level
         zalpha.2 <- -qnorm(alpha/2)
         gamma <- zalpha.2^2 * var.b1 / (b1^2)

         EC50 <- -b0/b1

         const1 <- (gamma/(1-gamma))*(EC50 + cov.b0.b1/var.b1)

         const2a <- var.b0 + 2*cov.b0.b1*EC50 + var.b1*EC50^2 -
                    gamma*(var.b0 - cov.b0.b1^2/var.b1)

         const2 <- zalpha.2/( (1-gamma)*abs(b1) )*sqrt(const2a)

         LCL <- EC50 + const1 - const2
         UCL <- EC50 + const1 + const2

         conf.pts <- c(LCL,EC50,UCL)
         names(conf.pts) <- c("Lower","EC50","Upper")

         return(conf.pts,conf.level,call=call)
 }

 EC50.calc(coefficients(fluor.fit),summary.glm(fluor.fit)$cov.unscaled)


 $conf.pts:
     Lower     EC50    Upper
  9.013366 9.538564 10.04191

 $conf.level:
 [1] 0.95


 $call:
 EC50.calc(coef = coefficients(fluor.fit),
           vcov = summary.glm(fluor.fit)$cov.unscaled)